Tertiary Phosphine Induced Migratory Carbonyl Insertion in Cyclopentadienyl Complexes of Iron

TERTIARY PHOSPHINE INDUCED MIGRATORY CARBONYL INSERTION IN CYCLOPENTADIENYL COMPLEXES OF IRON

NTAOLENG MAUREEN MAKUNYA

DISSERTATION

submitted in the fulfilment of the requirements for the degree

MASTER OF SCIENCE

CHEMISTRY

in the

FACULTY OF SCIENCE

at the

RAND AFRIKAANS UNIVERSITY

SUPERVISOR: PROF A. ROODT

JUNE 2004 ACKNOWLEDGEMENTS

I wish to express my sincere appreciation to Professor A. Roodt for introducing me into this field and for helping all the way through this venture unwearyingly. His dedicated group for their support, it did not go unnoticed.

Every thing was made possible by the kind assistance of Mr I. Foster, A. Muller, Dr. L. den Drijver, Dr. W. van Zyl, Dr. M. Haumann, Mrs Y. P. van Sittert, Mrs L. Rossouw, Mr. S. Mokhele and all my friends.

I would also like to thank my family for their support all the way.

TABLE OF CONTENTS

ABBREVIATIONS V ABSTRACT 1 1 INTRODUCTION 3 1.1. GENERAL 3 1.2. IRON AS A CATALYST 6 1.3. AIMS OF PROJECT 8

2 THE ROLE OF ORGANOMETALLIC CHEMISTRY IN CATALYSIS 10 2.1. INTRODUCTION 10 2.2. OVERVIEW OF IRON CHEMISTRY 10 2.2.1. Co-ordination in Organoiron Complexes 11 2.2.1.1. Iron Carbonyl Compounds 11 2.2.1.2. Carbido-clusters 13 2.2.2. Ferrocene and its Derivatives 15 2.2.3. Co-ordination in Bioinorganic Complexes 18 2.2.3.1. Iron Porphyrin Proteins 18 2.2.3.2. Non-Heme Iron Proteins 19 2.2.3.3. Superoxo Reductase 20 2.3. THE ROLE OF CATALYSIS 20 2.4. ACTIVATION OF MOLECULES 22 2.4.1. Ligand Co-ordination and Dissociation 23 2.4.1.1. Steric and Electronic Influences of Ligands on the Process 23 2.4.2. Oxidative Addition 25 2.4.2.1. Concerted Addition 27

2.4.2.2. SN2 Reactions 28 2.4.2.3. Radical Mechanism 29 2.4.2.4. Ionic Mechanism 30

2.5. PROXIMITY INTERACTION 30 2.5.1. Migratory Insertion Reaction 30 2.5.2. Mechanism and Rate Law of the Reaction 33 2.5.3. Factors Affecting Migratory Insertion 37 2.5.3.1. Ligand Effects 37 2.5.3.1.1. Steric Effects 37 2.5.3.1.2. Electronic Effects 38 2.5.3.2. Nature of the Metal 41 2.5.3.3. Lewis Acid 41 2.5.4. Reductive Elimination 42 2.6. APPLICATION IN CATALYSIS 43 2.6.1. Hydroformylation 43 2.6.2. Monsanto Process 46 2.6.2.1. Monsanto Technology 46 2.6.2.2. Function of the Iodide Salt 48 2.6.2.3. BP Low-water Technology (Cativa Process) 48

3 SYNTHESIS AND CHARACTERIZATION OF IRON COMPLEXES 50 3.1. INTRODUCTION 50 3.2. SYNTHESIS AND SPECTROSCOPIC CHARACTERIZATION OF THE COMPLEXES 50 3.2.1. General Considerations 50 5 3.2.2. Synthesis of [(η -C5H5Fe(CO)2)2)], [1] 51 5 3.2.3. Synthesis of [(η -C5H5)Fe(CO)2Me], [2] 53 5 3.2.4. Synthesis of [(η -C5H5)Fe(CO)(COMe){PPh3}] 54 5 3.2.5. Synthesis of [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}], A 56 5 3.2.6. Synthesis of [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}], B 56 5 3.2.7. Synthesis of [(η -C5H5)Fe(CO)(COMe){PPhCy2}] 57 5 3.2.8. Synthesis of [(η -C5H5)Fe(CO)(COMe){P(p-MeOPh)3}] 58 3.3. CHARACTERIZATION OF SELECTED COMPLEXES BY X-RAY CRYSTALLOGRAPHY 58 3.3.1. Theoretical Aspects 59 3.3.1.1. Basic Concepts in Crystallography 59

II 3.3.1.2. Structure Factor Fhkl 61 3.3.1.3. Fourier Synthesis 63 3.3.1.4. Patterson Maps 63 3.3.1.5. Least Squares Refinement 64 3.3.1.6. Difference Synthesis 65 5 3.3.2. Structure Determination of [(η -C5H5)Fe(CO)(COMe){P(p- 5 MePh)3}], A, and [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}], B. 66 3.3.2.1. General 66 3.3.2.2. Data Collection 66 3.3.2.3. Results of Refinement 67 5 3.3.2.4. The Crystal Structure of [(η -C5H5)Fe(CO)(COMe){P(p- 5 MePh)3}], Acetyl(carbonyl)(η -cyclopentadienyl)tri(p-tolylphosphine)- iron(II) 69 5 3.3.2.5. The Crystal Structure of [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}], Acetyl(carbonyl)(η5-cyclopentadienyl)tri(p-fluorophenylphosphine)- iron(II) 72 3.3.3. Structural Correlations 77 3.3.3.1. Correlation Between Structure of A versus B 77 3.3.3.2. Correlation with Literature Results 80 3.4. CORRELATION OF STRUCTURAL DATA WITH SPECTROSCOPIC FINDINGS 83

4 KINETICS AND MECHANISM OF THE MIGRATORY CARBONYL 5 INSERTION IN [(η -C5H5)Fe(CO)2Me] 86 4.1. INTRODUCTION 86 4.2. RATE LAWS AND REACTION ORDERS 87 4.3. ACTIVATION PARAMETERS 89 4.4. EXPERIMENTAL PROCEDURE 91 4.4.1. General Data 91 4.4.2. Kinetic and Spectroscopic Studies 92 4.5. MECHANISTIC ASPECTS AND RATE LAW 95 4.5.1. Basic Arguments for Mechanistic Scheme and Rate Law 95 4.5.2. Results and Discussion 100

III 4.5.2.1. Insertion Induced by Tri-p-tolylphosphine P(p-MePh)3 101 4.5.2.2. Insertion Induced by Tri-p-fluorophenylphosphine

P(p-FPh)3 106

4.5.2.3. Insertion Induced by Triphenylphosphine PPh3 119

4.6. CORRELATION OF DATA FOR PX3 (X = (p-FPh) and (p-MePh)) 120 4.7. RATE LAW FOR THE PROPOSED REACTION SCHEME 121 4.7.1. Possible Schemes for the Reaction 121 4.7.2. Formation of an Outer-sphere Intermediate Species (Scheme 4-1) 122 4.7.3. Formation of an Acyl Intermediate Species (Scheme 4-2) 124 4.7.4. Proposed Final Mechanism and Rate Law 126 4.7.5. Conclusion 127

5 EVALUATION OF RESULTS AND FUTURE RESEACH 128 5.1. SCIENTIFIC EVALUATION OF THIS STUDY 128 5.2. FUTURE RESEARCH 130

APPENDIX 132 5 A.1. CRYSTAL DATA OF [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}] 132 5 A.2. CRYSTAL DATA OF [(η -C5H5)Fe(CO)(COMe)P(p-FPh)3] 135 A.3. CALCULATION OF THE EFFECTIVE CONE ANGLE 138 A.4. EQUATIONS OF LEAST-SQUARES PLANES IN COMPLEX A AND B142 A.5. DERIVATION OF THE RATE LAWS 143 A.5.1. Formation of the Outer-sphere Intermediate Species 143 A.5.2. Formation of the Acyl Intermediate Species 146 A.6. SUMMARY OF METHODS USED TO DRY SOLVENTS 148 A.6.1. Dichloromethane 148 A.6.2. Acetonitrile 148

A.7. SUPPLEMENTARY DATA FOR P(p-MePh)3 149

A.8. SUPPLEMENTARY DATA FOR P(p-FPh)3 151 A.9. SPECTROSCOPY DATA 156

IV ABBREVIATIONS

AO = Acid optimisation Alumina = Aluminium oxide Ph = Phenyl Cp = Cyclopentadienyl DCP = Dicyclopentadiene FT-IR = Fourier-transform infrared IR = Infrared kobs = Observed pseudo first order rate constant L = Ligand [L] = Concentration of the ligand Me = Methyl group NMR = Nuclear magnetic resonance Cy = cyclohexyl

PX3 = tertiary phosphine QALE = quantitative analysis of ligand effect Tbp = Trigonal bipyramidal THF = Tetrahydrofuran UV-visible = Ultra violet-visible ABSTRACT

The aim of this study was to investigate the mechanism of phosphine induced migratory carbonyl insertion in the monocyclopentadienyliron(II) carbonyl complex, [η5-

(C5H5)Fe(CO)2Me], upon variation of different parameters such as the type and the concentration of the phosphine ligand and the solvent. The mechanism that agrees with the results obtained is presented below.

5 Characterization of the products of the reaction, [η -(C5H5)Fe(COMe)(CO){P(p-MePh)3}], A, 5 and [η -(C5H5)Fe(COMe)(CO){P(p-FPh)3}], B by X-ray crystallography, shows that both the complexes crystallize in the triclinic crystal system and space group P1 with R = 4.0 and 4.3 % for A and B respectively. The Fe-P and M-(C≡O) bond distances, which should be sensitive to the electron density around the metal centre are the same within the estimated standard deviation [(M-PA = 2.193(8) and M-PB = 2.194(8) Å while M-(C≡O) = 1.746(3) and

1.748(3) Å for both A and B respectively]. The effective cone angle of P(p-MePh)3 in A is

152.6 ˚ and for P(p-FPh)3 in B 152.4 ˚. This is larger than the classic Tolman cone angle in literature, ustilising Ni-P values of 2.28 Å, since the Fe-P bond distances are shorter. The variation in the angle P-Fe-Centroid angles are 126.1(1) ˚ and 127.4(1) ˚ for A and B respectively. The dihedral angles between plane 1 (plane described by Cp ring) and plane 2 [plane described by P, C(1) and C(2)] is 6.9(2) ˚ for A and 8.1(2) ˚ for B.

5 [η -(C5H5)Fe(CO)2Me] undergoes migratory carbonyl insertion in the presence of the PX3 ligands [X = (p-MePh) and (p-FPh)]. Two distinct reactions, as depicted by the Scheme below, were observed in MeCN for P(p-FPh)3, but not for P(p-MePh)3.

ABSTRACT

K1, k1 [(η5−C5H5)Fe(CO)2(Me)] + PX3 [(η5−C5H5)Fe(CO)2(Me), PX3]# k-1 k2

5 [(η −C5H5)Fe(COMe)(CO){PX3}]

Due to the fact that the reactions were very slow, the quality of the data, as well as the reproducibility thereof, was not that good and very large variations in rates have been obtained for some kinetic runs. However, the data could still be used to derive a reasonable mechanism for the overall reaction.

The reactants and products were characterised by NMR and IR. The calculated equilibrium -1 constant in MeCN at 50 ˚C for P(p-MePh)3 is 300 M (fixed during the least squares fit since -1 the data showed large uncertainties) compared to 36.3 ± 14.0 M for P(p-FPh)3.

By increasing the electron donating properties of the phosphine ligand from [P(p-FPh)3 to

P(p-MePh)3], an approximate order of magnitude increase in the reactivity, as depicted by the following rate constants obtained in MeCN at 50 ˚C, was observed: P(p-FPh)3 (k2 = (2.2 ± -6 -5 -1 0.2) x 10 ) and P(p-MePh)3 (k2 = (3.4 ± 0.4) x 10 s ).

Switching from MeCN (DN = 14.1 and ε = 37.5) to DCM (DN = 4 and ε = 9.1), showed a -5 -6 -1 decrease in the rate, i.e., from k2 = (3.4 ± 0.4) x 10 to (2.2 ± 0.5) x 10 s for P(p-MePh)3.

2 1 INTRODUCTION

1.1. GENERAL

Iron is the most abundant transition metal. It is quite inexpensive compared to other metals and is used in an increasing number and variety of organometallic catalysts. Iron is also relatively non-toxic in comparison to other transition metals used for catalysis [1]. According to Thomas [2] the use of an iron-based catalyst (Fe3O4) in the Haber process in 1909 was a benchmark in the history of applied catalysis. It is still arguably the best commercial catalyst, even though more than 100 000 other formulations have been investigated [2].

Iron, (Fe), is a metallic chemical element known since antiquity. Biblical records show that it was used in the making of tools as early as the fourth millennium BC [3], [4]. The symbol is derived from a Latin word for the metal, ferrum. It is a white lustrous metal that melts at 1528 ˚C and occurs in the native state in limited quantities in rocks and in meteorites, which are generally more than 90 percent iron. It is widely distributed as a constituent of rocks in the form of wuestite (FeO), hematite (Fe2O3) and magnetite, Fe3O4, of which the red-brown form is the most common. It is fourth in abundance among the elements of the earth’s crust and makes up most of the earth’s core [5], [6].

1 Mayer M. F., Hossain M., posters online. 2 Thomas J. M., Thomas W. J., “Principles and Practice of Heterogeneous Catalysis”, VCH Publishers, Inc., Weinheim, 1997, Chapter 1. 3 Watchtower Bible and Tract Society of Pennsylvania International Bible Students Association Copyright ©, “Insight of the Scriptures”, Vol. 1, Watchtower Bible and Tract Society of New York, Inc., USA, 1988, 1215. 4 There was a time in the history of man referred to, as Iron Age when iron made tool were popular. (Hornby S. A., “Oxford Advanced Learners Dictionary”, Oxford University press, New York, 2000, 634). 5 Bochman M., Cotton F. A., Murillo C. A., Wilkinson G., “Advanced Inorganic Chemistry”, 6th Ed., John Wiley & Sons, Inc., USA, 1999, Chapter 17. 6 Cotton F. A., Wilkinson G., Gaus P. L., “Basic Inorganic Chemistry”, 3rd Ed., John Wiley & Sons, Inc., USA, 1995, Chapter 24.

CHAPTER 1 INTRODUCTION AND AIM OF STUDY

Smelt processing of iron is done at high temperatures and requires large quantities of air. Two similar methods are involved, but in one there is deliberate slagging of the ore.

Materials such as limestone, CaCO3, are added as a flux for complexing SiO2-containing impurities. The reaction that takes place is with active CaO after CO2 is driven off. The charge of iron ores (Fe2O3, Fe3O4), coke (C) and limestone is heated with a blast of hot air. Combustion of coke in this air raises the temperature to 2000 °C and carbon burns to carbon monoxide in the lower part of the furnace [7].

The Fe2O3 from the top of the furnace reacts with the hot CO rising from bottom of the furnace. Iron(III) oxide is reduced, first to Fe3O4 and then to FeO at 500-700°C according to Eq. (1-1) and (1-2) below.

3Fe2O3 + CO → 2Fe3O4 + CO2 (1-1)

Fe3O4 + CO → 3FeO + CO2 (1-2)

On the other hand, CO is oxidised to CO2. At the same time CaCO3 is converted to lime

(CaO), so increasing the CO2 content of the exhaust gases.

CaCO3 → CaO + CO2 (1-3)

The CaO combines with the silicates present in the ore to form a layer of slag. The final reduction of iron occurs between 1000 and 1200 °C in the central region of the furnace as indicated in Eq. (1-4) and (1-5) below.

CaO + SiO2 → CaSiO3 (1-4)

FeO (s) + CO (g) → Fe (s) + CO2 (g) (1-5)

Iron, which contains high carbon content, is then drawn from the bottom of the furnace.

7 Shriver D. F., Atkins P. W., Langford C. H., “Inorganic Chemistry” 2nd Ed., Oxford University Press, Oxford, 1994, Chapter 7.

4 CHAPTER 1 INTRODUCTION AND AIM OF STUDY

Transition metal complexes catalyse a wide range of chemical transformations, some of which, like hydroformylation, polymerisation and hydrogenation, are processes of major industrial importance [8]. In transition metal catalysed reactions of organic substrates, it is the chemistry of the metal centre that determines the course of the reaction and not the substrate itself [9]. Therefore, transition metal complexes are studied as part of a general scheme to identify systems that might be used as homogeneous catalysts, catalyst precursors or catalyst models.

A catalyst accelerates the rate of a thermodynamically feasible reaction, opening a lower activation energy pathway. If an alternative route exists, a catalyst can enhance product selectivity by accelerating one of the competing reaction sequences. A catalytically active system must possess a vacant coordination site or must be able to generate it in a primary dissociation step. Thus, the reactant partners are brought in close proximity and also become activated for subsequent reaction on coordination to the transition metal. Not only should a catalyst bind substrates, but it should also be able to release the products.

In Homogeneous catalysis, whereby the catalyst is present in the same phase as the reagent, the coordination site is on the metal. Under normal circumstances, the catalyst is stable in more than one coordination state and through fine-tuning of chemical bond strength it is capable of holding substrate molecules selectively but not too tightly. This can be manipulated by ligand variation. In Heterogeneous catalysis the catalyst is present in a different phase from that of the reactants [7] and the vacant coordination site is located at a phase boundary, that is, only the surface atoms are catalytically active. Because of the simplicity of manipulating the homogeneous catalyst by ligand variation, it is easier to understand the mechanism of catalysis of a homogeneous catalyst than that of the heterogeneous counter-part. However, in industry, heterogeneous catalysis is more often used than homogeneous systems because of the ease of separation from the products. Latest developments in catalysis have led to combining the heterogeneous and homogeneous catalyst to give a catalyst that retains the good properties while eliminating the unwanted features.

8 Pfister B., Stauber R., Salzer A., J. Organomet. Chem., 1997, 553, 131. 9 Hegedus L. S., “Transition Metals in the Synthesis of Complex Organic Molecules”, University Science Books, USA, 1999, Chapter 2. 5 CHAPTER 1 INTRODUCTION AND AIM OF STUDY

1.2. IRON AS A CATALYST

Iron can be used as a catalyst in its elemental form or as organo-iron compounds. The organo-iron compounds that are used in pyrolysis decompose during the process, forming elemental iron, which is the actual catalyst, used in hydrogenation and dehydrogenation reactions. Iron is also known for its strong sulphur affinity; consequently organically bound sulphur in tars and pitches can be transformed to iron sulphide, FeSx. This reaction limits the lifetime of an iron catalyst (“catalytic poisoning”), but with beneficial side effects because inorganically bound sulphur is less problematic than heterocyclically bound sulphur [10].

The rapid, and at times drastic, increase in the price of crude oil, coupled with the realization that crude oil supplies are likely to fall short of potential demand, rekindled interest in coal both as a source of energy and as a feedstock for the petrochemical industry. In order to use coal as a feedstock, it has to be converted into gas, liquid hydrocarbon products and preferably discrete organic chemicals. One way is by the use of the Fischer-Tropsch synthesis, in which synthesis gas, a mixture of hydrogen (H2) and carbon monoxide, (CO) (produced by burning coal in the presence of steam) is converted to a wide range of hydrocarbon products in the presence of iron or cobalt catalyst [11].

As a basic technology for producing synthetic liquid fuels from CO + H2 feed streams catalytic synthesis processes have undergone worldwide development since the 1920s. Iron– based catalysts have been investigated and precipitated iron catalysts and fused iron catalysts have been commonly used in the Fischer-Tropsch (F-T) process. Because of the need for improvement of the activity and selectivity of iron catalysts for F-T synthesis processes, development of an improved skeletal iron catalyst was initiated [12].

To date, iron-based catalysts are still used in South Africa to produce gasoline, [13] with 40- 50 % of the entire South African supply produced with this process. Each year about 22 million tons of coal is converted to at least 4.5 million tons of gasoline. The nature of the

10 Müller W. M., Hoffmann R. W. Hüttinger K. J., Fuel 1995, 74, 7, 953. 11 Masters C., “Homogeneous Transition Metal Catalysis; a Gentle Art”, Chapman & Hall Ltd; 1981, Chapter 3. 12 Zhou J., Lu Y., Zhang Z., LI G., Dong L., Wang H., Zhou P., Lee L., U.S. Patent, 6,265,451, 2001, 1. 13 Cornils B., Herrmann W. A., “Applied Homogeneous Catalysis with Organometallic Compounds”, Vol. 2, Wiley-VCH, Weinheim, 2002, Chapter 3.

6 CHAPTER 1 INTRODUCTION AND AIM OF STUDY catalyst and various reaction parameters determine the final product pattern. Kolbel Engelhardt variants (see the equation below) yield up to 60 % of hydrocarbons and 40 % of oxygenates such as alcohols, aldehydes and other oxygen containing products. This process makes use of an iron-based catalyst at 180-280 ˚C, medium pressure, and a CO-containing gas mixture.

cat 3nCO + nH2O → (CH2)n + 2nCO2 (1-6)

With an iron based precipitation catalyst at 210-250 ˚C, mostly higher-boiling hydrocarbons are manufactured by means of Ruhrchemie/Lurgi technology. Low-boiling products arise from the Kellogg–Synthol process, still using iron, but at higher temperatures (300-340 ˚C). In the classical Synthol and Kogasin processes of Fischer-Tropsch, the reducing action of water leads to the formation of carbon dioxide that is in equilibrium under standard process conditions. The water gas shift reaction also makes use of an iron-based catalyst, but the most - efficient is the rhodium complex [Rh(CO)2I2] , the key catalytic species in the Monsanto process [14].

Even though iron has found great industrial application as a heterogeneous catalyst, it is widely used in nature as a homogeneous catalyst. It is at the active centre of molecules responsible for oxygen transport and electron transport. It is found in or with such diverse metalloenzymes as various oxidases, hydrogenase reductases, dehydrogenases deoxygenases and dehydrases. Fe is found in the whole gamut of life forms from bacteria to man [5]. Some of these enzymes will be discussed briefly in Chapter two.

Against this background, this study was designed to investigate different synthetic and reactivity studies on carbonyl metal complexes and various ligands.

14 Cornils B., Herrmann W. A., “Applied Homogeneous Catalysis with Organometallic Compounds”, Vol. 3, Wiley-VCH, Weinheim, 2002, Chapter 3.

7 CHAPTER 1 INTRODUCTION AND AIM OF STUDY

1.3. AIMS OF PROJECT

Transition metal alkyl and acyl compounds have been widely studied because of their fundamental importance in chemistry and their significance as models for intermediates in a wide variety of industrially important catalytic reactions [15].

In this study we aimed to investigate the activation of the carbonyl ligand. We selected our model complex to be monocyclopentadienyliron(II) carbonyl complexes, [(η5-

C5H5)Fe(CO)2Me]. These half sandwich alkyl complexes are used because of the presence of a M-C (carbonyl) bond, the properties of which can readily be studied by FT-IR spectroscopy. The simplicity of these complexes’ IR spectra makes it easy to study the reaction progress without interference of overlapping peaks. In addition to the CO moieties there is one cyclopentadienyl ring and a methyl group, which enable the use of 1H NMR to evaluate the electronic properties of the complexes. These model complexes can undergo migratory carbonyl insertion to form the acyl species. The whole process will be discussed in detail in Chapter two, and the kinetic and mechanistic aspects of this will be discussed fully in Chapter four. A range of phosphine ligands was used to induce the insertion enabling the use of both phosphorus and fluorine NMR to quantitatively study the progress of the reaction. Phosphine ligands characteristically have variable electronic and steric properties [16] and are therefore ideal for this study and catalysis as a whole.

Since transition metals including iron can attain different oxidation states they therefore have reduction/oxidation potentials and thus electrochemistry can be used to study the model complexes.

The acyl complexes are crystalline and consequently characterisation by X-ray crystallography can also be used to evaluate both the electronic and steric properties of different ligands in the complex.

The monoanionic cyclopentadienyl ring acts as a tridentate ligand and occupies three coordination sites on the metal, while the remainder of the ligands occupy the remaining

15 Anderson J. M., Moss J. R., J. Organomet. Chem., 1995, 494, 105. 16 Tolman C. A., Chem. Rev., 1977, 77, 3, 313. 8 CHAPTER 1 INTRODUCTION AND AIM OF STUDY coordination sites. The complex is chiral and the isomers can be resolved. The protons α to the acyl group are acidic and the corresponding metal acyl enolate undergoes reaction with a variety of electrophiles. More interestingly, because the complex is chiral at the iron and one face is hindered by the phosphine ligand, the reactions of these acyliron enolates occur with very high stereoselectivity. This chemistry has been used extensively in the synthesis of optically active compounds, [9] which is beyond the scope of this study.

With the above in mind the following aims were set out for this study.

To synthesise a series of cyclopentadienyl iron carbonyl complexes with different tertiary phosphine ligands (triphenylphosphine, tri(p-tolyl)phosphine, tri(p-fluorophenyl)phosphine, tri(p-methoxyphenyl)phosphine and dicyclohexylphenyl- phosphine).

To evaluate the nucleophilicity (electron donor ability) of different ligands on manipulation of the iron centre, with special reference to the P-atom in various tertiary aryl- and alkyl phosphine ligands using IR and NMR.

To study solid state properties of acetylcarbonyl(η5-cyclopentadienyl)tri(p-fluorophenyl)phosphineiron(II) and acetylcarbonyl(η5-cyclopentadienyl)tri(p-tolyl)phosphineiron(II) complexes using X-ray crystallography to characterize their crystal structures and elucidate the effect induced by the different p-substituted aryl phosphines on the crystal structure.

To study the kinetics of coordination of ligands to the metal centre (Eq. (1-1) using IR, NMR and UV-visible and deduce the mechanism that is followed during the reaction.

5 5 [(η -C5H5)Fe(CO)2Me] + L —→ [(η -C5H5)Fe(COMe)(CO){L}] (1-1)

Here L is P(p-FPh)3 and P(p-MePh)3.

9 2 THE ROLE OF ORGANOMETALLIC CHEMISTRY IN CATALYSIS

2.1. INTRODUCTION

The first organometallic compounds ([PtCl2(C2H4)]2 and K[PtCl3(C2H4)]) were prepared in 1827 by Zeise [1]. Modern organometallic chemistry of transition metals began with the synthesis of the cyclopentadienyl complex ferrocene, (C5H5)2Fe, in 1951. Since that time cyclopentadienyl complexes of a wide variety of metals, including transition elements, have been prepared [2]. Recently metallocene systems have been used as catalysts in important inorganic and organic reactions. For example, Mono- and bis(cyclopentadienyl) complexes of early transition metals are now well established as important olefin polymerisation catalysts [3]. In this study, half sandwich carbonyl complexes of iron were studied.

2.2. OVERVIEW OF IRON CHEMISTRY

Iron is a 3d element that belongs to group VIII of the periodic table. It has the chemical symbol Fe taken from its Latin name, Ferrum. Chemically, iron is a fairly reactive metal forming two series of salts: ferrous and ferric. In ferrous compounds the metal has an oxidation state of II, while ferric has an oxidation state of III [4]. According to Cotton et al. [5] these are the only oxidation states of importance in the ordinary aqueous and related chemistry of iron. The highest oxidation state known is VI and is very rare. In moist air iron

1 Douglas B. E., MacDaniel D H., Alexander J. J., “Concepts and Models of Inorganic Chemistry”, 3rd Ed., John Wiley & Sons, Inc., New York, 1974, Chapter 12. 2 Angelici R. J., “Synthesis and Technique in Inorganic Chemistry”, 2nd Ed., University Science Books, Mill Valley, California, 1977, Experiment 15. 3 Galindo A., Gómez M., del Rĺo D., Sánchez F., Eur. J. Inorg. Chem., 2002, 1326. 4 Cotton A. F., Wilkinson G., Gaus P. L., “Basic inorganic chemistry”, 3rd Ed., John Wiley & Sons, Inc., USA, 1995, Chapter 24. 5 Cotton F. A., Wilkinson G., Murillo C. A., Bochmann M., “Advanced Inorganic Chemistry”, 6th Ed., John Wiley & Sons, Inc., USA, 1999, Chapter 17.

CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS is rapidly oxidized to rust (hydrous iron oxide) that flakes off, exposing fresh metal surfaces. Finely divided Fe is pyrophoric. It reacts vigorously with halogens and a number of non- metals such as sulphur, phosphorus, bismuth, carbon and silicon. The carbide and the silicide phase play a major role in technical metallurgy of iron as has been shown in the previous chapter [5].

2.2.1. Co-ordination in Organoiron Complexes

Organometallic chemistry of iron is dominated by reactions with π acid ligands such as carbonyl (CO) and cyclopentadienyl (Cp) derivatives [5].

2.2.1.1. Iron Carbonyl Compounds

Iron forms three types of homoleptic carbonyls, e.g. pentacarbonyliron, Fe(CO)5, yellow- orange solid enneacarbonyldiiron , [Fe2(CO)9] and black solid dodecacarbonyltriiron

([Fe3(CO)12]). Fe(CO)5 has a melting point of 20 ˚C and a boiling point of 101 ˚C. -5 [Fe2(CO)9] sublimes at 10 ˚C / 10 mm and [Fe3(CO)12] at 70 ˚C / 0.11 mm. The characteristic feature of these complexes is the presence of a metal-carbon monoxide bond.

Fe(CO)5 has a slightly distorted trigonal bipyramidal (tbp) structure with the Fe-C distances very similar (ca. 1.80 – 1.81 Å at 198 K). It is easily prepared by reaction of elemental iron with CO at high temperature and pressure. It decomposes into elemental iron and CO and is a precursor to nearly all other iron carbonyl derivatives.

Irradiation of Fe(CO)5 with ultraviolet light promotes CO elimination, forming [Fe2(CO)9]

[6]. The characteristic features of [Fe2(CO)9] are three bridging carbonyl groups and an Fe-Fe bond. Heating [Fe2(CO)9] to slightly above room temperature causes it’s dissociation into

Fe(CO)5 and Fe(CO)4 fragments according to the following equation:

6 Crabtree R., “The Organometallic Chemistry of the Transition Metals”, 3rd Ed., John Wiley & Sons, Inc., USA, 2001, Chapters 4, 12, 13.

11 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

Fe2(CO)9 → Fe(CO)5 + Fe(CO)4 (2-1)

The unstable Fe(CO)4 fragment reacts with other species present in solution to form substituted iron carbonyl derivatives or trimerizes forming [Fe3(CO)12] [7].

Dodecacarbonyltriion [Fe3(CO)12] occupies a special place in the carbonyl cluster chemistry. Braga et al. [8] found that the cluster undergoes isomerization in solution at room temperature. The IR spectrum consists of two strong bands in the νCO stretching region and two weak bands in the νCO bridging region. Only at 20 K in the presence of an argon matrix is the IR spectrum consistent with the presence of two bridging and ten terminal CO ligands. Their analysis of iron K-edge extended X-ray absorption fine structure (EXAFS) data with multiple scattering showed that [Fe3(CO)12] in light petroleum is present mainly as the terminal structure, whereas in the more polar medium of a frozen CH2Cl2 solution the bridged structure is favoured.

The chemistry of the carbonyl compounds, especially Fe(CO)5, is dominated by reactions in which there are dissociative losses of one or more CO ligands, followed by complexation and / or oxidative addition or nucleophilic attack at the carbon [5].

130 - 140 °C [(η5−C H Fe) (CO) ] + 6CO + H 2FeCO5 + C10H12 5 5 2 2 2 oxidised I (Fe0) (Fe )

2 Na/Hg Reduced

5 2 CH3I 5 - + 2[(η −C5H5)Fe(CO)2CH3] 2[(η −C5H5)Fe(CO)2] + 2 Na oxidised 0 (FeII) (Fe )

Migratory insertion + 2L No change in oxidation number

5 2[(η −C5H5)Fe(COCH3)(CO){L}]

Scheme 2-1 Typical reaction of [Fe(CO)5] [2].

7 King B. R., “Transition-Metal Organometallic Chemistry: an Introduction”, Academic Press Inc., New York, 1969, Chapter 5. 8 Braga D., Grepioni F., Farrugia J. L., Johnson F. G. B., J. Chem. Soc. Dalton Trans., 1984, 2911.

12 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

2.2.1.2. Carbido-clusters

An early stimulus to cluster chemistry was the cluster-surface analogy, which proposed that cluster chemistry would resemble the surface of metals because both surfaces and clusters consist of arrays of metal atoms. However, it was found that organic compounds bind to clusters differently than to single metals. These new structures provide important clues for surface chemistry where direct structural data for surface-bound species are still very hard to obtain [6].

The earliest cluster to be recognized was [Fe5(CO)15]. The fully encapsulated carbon (Scheme 2-2) atoms are relatively unreactive and it is the smaller four-iron cluster, 2- 2- [Fe4(CO)13] that has the greatest chemical activity. [Fe4(CO)13] was first synthesized as the 2+ [Fe(C5H5N)6] salt in 1930 and its protonated derivative was reported about 30 years later. 2- X-ray crystal structure determinations established that [Fe4(CO)13] consists of a tetrahedral - array of iron atoms and that [Fe4(CO)13H] contains a butterfly array of iron atoms with a bridging carbonyl (µ-CO) group. Subsequent variable-temperature multinuclear NMR - spectroscopic studies indicate that [Fe4(CO)13H] exist in solution as an equilibrium mixture of two isomers [9] as shown in Eq. (2-2).

- - O Fe C Fe Fe Fe Fe Fe Fe Fe A A

Tetrahedron Butterfly 2a 2b (2-2)

A bridging CO, µ-CO, such as that in 2b, is postulated to be important in the cleavage of CO on metal surfaces; Shriver and Whitmire [10] demonstrated that in acid solution the µ-CO in

9 Wang J., Sabat M., Horwitz C. P., Shriver F. D., Inorg. Chem., 1988, 27, 552. 10 Whitmire K., Shriver D. F., J. Am.Chem. Soc., 1980, 102, 1456.

13 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

2b is converted to CH4.

The exposed carbon atom in the Fe4C system can be thought of as a possible model for a surface carbon atom in heterogeneous catalytic processes. According to Herrmann and Cornils [11] the sterically exposed carbon atom in the oligonuclear iron cluster structure of

[Fe4(CO)13] undergoes easy reversible hydrogenation by intramolecular hydrogen migration. 2- The carbide cluster, [Fe6(µ6-C)(CO)16] , can be prepared by reduction of metal carbonyls [12]. Reactivity of the carbide could not be studied because it is concealed in the cluster. Scheme 2-2 below shows how the cluster can be opened up by controlled oxidation to give a reactive Fe4C with the carbon in a four-coordinate “butterfly” environment [13].

C7H7Br Oxidation C C + Br-

[Fe ( C)(CO) ]2- [Fe6(µ6-C)(CO)16]2- 5 µ5− 15 Oxidation -FeBr2

C C H H+

H [Fe4(µ4−C)(CO)12]2-

2-1 Oxidation CO C7H7Br MeO O C C C MeOH

vacuo

[Fe4(µ4−C)(CO)13] [Fe4(µ4−C)(CO)12(CO2Me)]- 2-2

CH3COOMe

Scheme 2-2 Formation of a reactive carbido-cluster by oxidation and protonation [6], [12], [13].

11 Cornils B., Herrmann W. A., “Applied Homogeneous Catalysis with Organometallic Compounds”, 2nd Ed., Vol. 2, Wiley-VCH, Weinheim, 2002, Chapter 3. 12 Adams R. D., Horváth I. T., Prog. Inorg. Chem., 1983, 33, 127. 13 Bradley J. S., Adv. Organometal. Chem., 1983, 22, 1.

14 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

Metal carbides undergo substitution and fragmentation with both processes equally favoured as illustrated in Eq. (2-3) below.

Fe3(CO)12 + PPh3 → Fe3(CO)11PPh3 + … + Fe(CO)5 +… + Fe(CO)3(PPh3)2 + CO (2-3)

Metal cluster anions are much stronger bases than their neutral analogues, thus there is a higher tendency of protons to attach to metal atoms in clusters than in mononuclear carbonyls [14].

2- + - [Fe3(CO)11] + H → [Fe3H(CO)11] (2-4)

2.2.2. Ferrocene and its Derivatives

Ferrocene is a cyclopentadienyl iron complex that has an iron atom “sandwiched” between two planar Cp rings (Figure 2-1 A). For this reason, bis(cyclopentadienyl)complexes are sometimes called “sandwich” compounds or metallocenes. The normal bonding mode for Cp is η5 (pentahapto). Several different resonance structures can be drawn for bonding of η5-Cp ligand to a transition metal complex [15]. This is illustrated in Figure 2-1 B.

(A)

M Fe M (B)

Figure 2-1 Schematic representation illustrating (A) iron sandwiched between two planar Cp rings and (B) other modes of bonding between Cp ring and transition metals in general [15].

14 Shriver D. F., Atkins P. W., Langford C. H., “Inorganic Chemistry”, 2nd Ed., Oxford University Press, 1994, Chapter 16 . 15 The Organometallic HyperTextBook, Cyclopentadienyl Ligands, http://www.ilpi.com/organomet/cp.htm.

15 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

Ferrocene is an orange crystalline, diamagnetic solid with a melting point of 173-174 ˚C. It is highly volatile but thermally stable, decomposing at 500 ˚C. It is also air stable in the solid state or when dissolved in organic solvents. Ferrocene can be prepared as illustrated in Eq. (2-5).

THF 2NaC5H5 + FeCl2 → (C5H5)2Fe + 2NaCl (2-5)

In ferrocene each Cp ligand donates six electrons to iron (d6-Fe) so that the eighteen-electron configuration is attained. Most Cp complexes display completely delocalised bonding with equivalent carbon-carbon bond lengths in the C5 ring. Ferrocene has longer C-C bond distances than other aromatic systems, and the longer C-C distance in ferrocene is the result of π-back bonding from the filled d-orbitals of Fe to the antibonding molecular orbitals on Cp [15].

The C5 ring of ferrocene is planar and the hydrogen atoms are bent downwards towards the metal by about 5 degrees. In decamethylferrocene where methyl (Cp*) groups replace the ferrocene protons the methyl groups are tilted above the C5 plane by 3.4 degrees due to steric interactions. The methyl groups on the Cp* ring are electron donors, so this results in more electron density on the metal than on the analogous Cp complex [15].

Another class of cyclopentadienyl complexes is the indenyl complexes, which have a benzene - ring attached to the Cp ring. The indenyl anion, C9H7 , is also a six-electron donor. The chemistry of the indenyl transition metal complexes shows enhanced reactivity widely known as the indenyl effect. The higher reactivity in kinetic studies has been explained on the basis of the special ability of the indenyl group to undergo a ring slippage from η5- to η3- coordination, which is favoured by generation of the aromaticity in the fused benzene ring [16]. This part will be discussed further with examples in section 2.4.1.1.

One Cp ring of the ferrocene can be substituted by other ligands and the resulting complexes are referred to as “half sandwich” complexes. In ferrocene chemical reactions take place at the Cp rings, but in half sandwich complexes the Cp ring act as a spectator ligand for a whole

16 Cadierno V., Dĺez J., Gamasa M. P., Gimeno J., Lastra E., Coord. Chem. Rev., 1999, 193-195, 147.

16 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS series of complexes CpMLn (n= 2, 3, 4), where interest lies in the chemistry occurring at the

MLn group. CpMLn are often referred to as “two-, three-, or four-legged piano stool” with Cp being regarded as the seat and the other ligands as the “legs” [6]. The monohapto-, trihapto- and pentahapto-Cp complexes, referred to as η1, η3, and η5 respectively, are known.

The Cp ring acts as a tridentate ligand and occupies three coordination sites and the rest of the ligands the remaining coordination sites. If different ligands are used the complexes become chiral at iron, and can be resolved. In the CpM(CO)(COMe)L complexes (where L is a phosphine ligand) the protons α to the acyl group are acidic and the corresponding metal acyl enolate undergoes reaction with a variety of electrophiles. More interesting, because the complex is chiral at the iron and one face is hindered by the phosphine ligand, the reaction of this acyliron enolate occurs with very high stereo selectivity [17]. This phenomenon has been used extensively in the synthesis of optically active compounds.

Transition metal alkyl and acyl compounds have been widely studied because of their fundamental importance in chemistry and their significance as models for metal alkyl and acyl intermediates in a wide variety of industrially important catalytic reactions [18]. The acyl complexes are formed by migratory insertion undergone by alkyl complexes. The whole process will be discussed in detail in section 2.5 and the rate at which the whole process takes place will be discussed fully in Chapter four. In their IR study of protonated alkyl and acyl complexes of this type, Green and Hurley [19] showed that the oxygen of the carbonyl group is fairly basic and therefore can be protonated. The protonated acyl complexes containing iron-carbene bonds are useful for carbene synthesis.

It has been demonstrated in the previous Chapter that iron is extensively applied in heterogeneous catalysis, but in nature, iron is widely used as a homogeneous catalyst. A few examples as well as some synthetic mimics of natural enzymes, which are widely applied in synthesis, are discussed below.

17 Hegedus L. S., “Transition Metal in the Synthesis of Organic Molecules”, University Science Books, USA, 1999, Chapter 5. 18 Anderson J. M., Moss J. R., J. Organomet. Chem., 1995, 494, 105. 19 Green M. H. L., Hurley C. R., J. Organomet. Chem., 1967, 10, 188.

17 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

2.2.3. Co-ordination in Bioinorganic Complexes

Iron is the active centre of molecules responsible for oxygen transport and electron transport. It is found in, or with such diverse metalloenzymes as various oxidases, hydrogenase reductases, dehydrogenases, deoxygenases and dehydrases. Fe is found in the whole gamut of life forms from bacteria to man [5]. In biological systems, there are three well-characterised iron systems:

2.2.3.1. Iron Porphyrin Proteins

The following proteins, haemoglobin, myoglobin and cytochrome P450, are briefly discussed. Iron porphyrins play a major role in oxygen binding in haemoglobin and myoglobin. Both haemoglobin and myoglobin have iron centres co-ordinated by four nitrogen atoms as their active site. Deoxy myoglobin is a five coordinate high spin iron(II) complex with four of the coordination positions occupied by the porphyrin nitrogen atoms. A nitrogen atom of the imidazole group of the histidine residue F8 occupies the fifth position. As with organometallic complexes that acts as catalysts facile co-ordination and release of substrates is necessary, thus the 5-coordination mode enables the reversible binding of molecular oxygen in the sixth co-ordination site as shown in Figure 2-2 below. In the deoxy form the iron(II) atom (high spin (s = 2) state) lies about 0.4 Å out of the heme plane in the direction of the histidine group. In the oxygenated form iron(III) ion is in the low spin state and is nearly centred in the porphyrin plane [14].

Protein Protein

N N

N N o 0.4 A N N N Fe Fe N N N N N O O Deoxy form Oxy form

Figure 2-2 The schematic representation of the deoxy- and oxy- form of the iron porphyrins [14].

18 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

The cytochromes are heme proteins that act as electron carriers, linking the oxidation of substrates to the reduction of oxygen. The co-ordination sphere of the iron consists of the porphyrin ring plane, one sulphur atom of a methionine residue occupying one side of the plane and a nitrogen atom of an imidazole ring occupying the other side. Oxygen binding is therefore prohibited. Cytochromes operate by shuttling iron atom between various oxidation states (FeII-FeV), the process converting C-H groups to C-OH groups according to Eq. (2-13) [4], [5]. One well-known example is cytochrome P450. This is a super family of cysteine thiolate-ligated heme iron enzymes that activate dioxygen for insertion or addition of a single oxygen atom into a wide variety of substrates, including alkanes to form alcohols and alkenes to form epoxides [20]. The P450 enzymes are critical to many biological processes including steroid hormone biosynthesis, drug metabolism and detoxification of xenobiotics. P450 has been characterised by X-ray crystallography and the knowledge gained enabled synthesis of several site-specific P450 mutants. One such mutant is T252A P450cam, which is able to catalyse epoxidation of olefins [20].

+ RH + O2 + 2e + 2H → ROH + H2O (2-6)

2.2.3.2. Non-heme Iron Proteins

These classes of proteins, iron-sulphur clusters, nitrogenase, rubredoxin and ferredoxins, contain strongly bound functional iron atoms, but no porphyrins [4]. Sulphur atoms bind iron atoms, the most important being those containing iron in a tetrahedral environment of four sulphur atoms. One or more iron atoms may be involved at the active site in iron-sulphur proteins. They all participate in electron transfer sequences. In rubredoxin proteins from bacterium clostridium pasteurianum the iron atom, FeIII, is surrounded by a distorted tetrahedron of cysteinyl sulphur atoms. The Fe-S distances range from 2.24 to 2.33 Å and the S-Fe-S angles from 104 to 114˚. When the FeIII is reduced to FeII there is a slight (0.05 Å) increase in the Fe-S distances but essential tetrahedral co-ordination is maintained [14].

20 Jin S., Makris T. M., Bryson T. A., Sligar S. G., Dawson J. H., J. Am. Chem. Soc., 2003, 125, 3406 (and references therein).

19 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

Another class of non-heme proteins is the small proteins containing iron-sulphur redox centres. Bonds from cysteine sulphur atoms (S) to iron hold clusters of two, three or four atoms together with several S atoms. In each case an approximate tetrahedron of sulphur is formed about each iron atom by sulphur atoms of cysteine residues of the peptide. Two iron ferredoxins with their attached cysteine sulphur atoms can be described as two tetrahedral n+ FeS4 units sharing an edge, represented as [2Fe-2S] (where n is preferably equal to two). In these units both iron atoms are iron(III), but can be reduced at –0.4 V (hydrogen scale) to [2Fe-2S]+. In this state the added electron is localised on one iron atom so that FeII and FeIII atoms are present. An Fe-Fe bond distance of 2.72 Å is observed in the [2Fe-2S]2+ cluster, which is also diamagnetic. After reduction the cluster has only one unpaired electron [4].

2.2.3.3. Superoxo Reductase

Superoxo reductase comprises a family of enzymes with an active site domain of 100 amino acids accommodating a single Fe ion. This ion is co-ordinated by four equatorial histidine nitrogens and an axial cysteinyl sulphur atom [21]. The active site is square pyramidal ferrous [Fe(NHis)4(SCys)]. The ‘resting’ ferric form of this site is pseudo-octahedral with a carboxylate from a glutamate occupying the sixth co-ordination position, trans to the cysteine ligand. The resting ferric superoxo reductase site exhibits ligand to metal charge transfer (LMCT) [22]. An enzyme found in several air sensitive bacteria and archaea catalyse the one electron reduction of superoxide to hydrogen peroxide at a unique square pyramidal ferrous

[Fe(site NHis)4SCys] site [22].

2.3. THE ROLE OF CATALYSIS

The role that catalysis play is best explained by the transformation it induces. Catalysts are

21 Auchere F., Raleiras P., Benson L., Tavares P., Muora J. J. G., Venyaminor S. Y., Moura I., Rusnak F., Inorg. Chem., 2003, 42, 938 (and references therein). 22 Silaghi-Dumitrescu R., Silaghi-Dumitrescu I., Coulter E. D., Kurtz, D. M. Jr., Inorg. Chem., 2003, 42, 446 (and references therein).

20 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS involved in processes in which substrates are transformed into products, which can be thermodynamically favourable or unfavourable. The position of the equilibrium is dependent on the difference in free energy, ∆G, between reactants and products.

∆G = Gproducts - Greactants (2-7)

At standard condition (25˚C or 298.15 K at 1 atm.) the standard free energy at equilibrium is given by Eq. (2-15), where Kp is the equilibrium constant expressed in terms of the partial pressures of the reactants and products.

∆G˚ = -RTlnKp (2-8)

For ∆G˚ < 0 the reaction is thermodynamically favourable, if 0 < ∆G˚< 40 kJ it is questionable but worth investigating whether the reaction will proceed, and if ∆G˚ > 40 kJ no catalytic reactivity is expected [23].

Thermodynamically favourable reactions might take place at a slow rate or not at all in the absence of a catalyst.

-1 H2(g) + 1/2O2(g) → H2O(g) ∆G˚ = -228.6 kJ mol (2-9)

40 -1/2 For reaction (Eq. (2-9)) shown above the Kp value of 1.19 x 10 atm shows that the equilibrium lies completely to the right. When H2 and O2 are carefully mixed in the pure state nothing happens, but when a small amount of catalyst is added the reaction takes place rapidly. A catalyst does not alter the final position of the equilibrium but only speeds up a reaction. For a thermodynamically favourable reaction a catalyst offers reactants an alternative, lower energy, pathway to products. Therefore, for a catalyst to be effective, it should be able to significantly decrease the standard free energy of activation that corresponds to the highest free energy barrier along the route from reactants to products [6]. It should bring about a reaction at a temperature below that required for uncatalysed thermal reaction,

23 Masters C., “Homogeneous Transition-Metal Catalysis: a Gentle Art”, Chapman and Hall Ltd, USA, 1981, Chapter 1.

21 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS which saves energy in commercial applications. It often gives higher selectivity for the desired product, minimising the need to discard the undesired product as waste. Environmental concerns have promoted the idea of atom economy, which values a process most highly when all the atoms in the reagent are being used to form the product minimising waste (for example, in the Monsanto process, all MeOH and CO is converted to MeCOOH) [6].

With the growing regulatory pressure to synthesise drugs in enantiopure form asymmetric catalysis has come to the fore, along with enzyme catalysis as the only practical way to make such products on a large scale [6]. One isomer may be a useful drug while the other isomer may be fatal. Consequently, a vast majority of products of chemical industry involve a catalyst at some stage in their manufacture for selectivity, preservation of energy, combating of pollution, and many other factors.

Most catalysed processes employ heterogeneous catalysts, which are easily separated from products. When special conditions, such as high selectivity and mild reaction conditions are required, homogeneous catalysts with their well-defined ligand systems and high chemical uniformity are preferred [5]. In addition to being selective for product formation, it should have a long lifetime to survive through a large number of cycles if it is to be economically viable.

Reviewing a few basic organometallic reaction types helps to understand catalytic reactions. These fundamental reaction steps involved in transition metal catalysed reactions will be discussed fully below to give an insight into the mechanistic principle of catalysis. The reactions will be divided into two sections, (I) those responsible for activation of molecules and (II) those responsible for bringing activated molecules together either to give a further activated intermediate [23] or to achieve suitable fictionalisation of the substrates with subsequent release of the products of the catalytic reaction [5].

2.4. ACTIVATION OF MOLECULES

Homogeneous catalysis proceeds via a series of organometallic reaction steps, coupled to one

22 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS another in such a way that they form a loop [24]. Some of these reactions are discussed to illustrate their use in catalytic processes.

2.4.1. Ligand Co-ordination and Dissociation

Co-ordination of ligands is one of the ways in which catalysts activate reactants and facilitate reactions. Therefore facile co-ordination of reactants to the metal ion and facile loss of products from the co-ordination sphere are required during catalysis to ensure that the reactions occur with minimum activation energy [14]. Co-ordinatively saturated catalyst precursors become active catalysts by ligand loss [5]. Solvated square planar d8 systems, which are naturally co-ordinatively unsaturated, are ideal for catalysis as the solvent is substituted by the incoming reactants.

solvent A L L4 1 L1 L4 M + A + B M + 2 solvent L2 L3 L L solvent 2 3 B

Scheme 2-3 The solvated co-ordinatively unsaturated species.

In five or six co-ordinated metal complexes co-ordination sites are created either by thermal or photochemical dissociation of one or more ligands, or by change in oxidation state at the metal centre with varying co-ordination modes [4]. The latter process will be discussed in section 2.4.2.

2.4.1.1. Steric and Electronic Influences of Ligands on the Process

The degree of dissociation to create a binding site is dependent on the σ-donor and π-acceptor

24 Elschenbroich Ch., Salzer A., “Organometallics: a Concise Introduction”, VCH Publishers, Weinheim, Germany, 1989, Chapter 17.

23 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS qualities of the ligand. Strongly basic phosphine ligands and good π-acceptor ligands such as CO do not dissociate readily, because of strong π-back acceptor bonds formed between the carbonyl carbon atom and the metal. In addition, the ligand size determines how many ligands a metal can accommodate, as well as how fast that complex is going to react. Smaller ligands lead to a high co-ordination number [5]. If these ligands are good π-acceptors the M- CO bond is weakened, hence facile dissociation is possible. On the other hand, if the coordinating ligand is a poor π-acceptor ligand, M-CO bond is strengthened and dissociation becomes difficult.

Ligands with varying co-ordination modes can slip to a lower co-ordination mode to accommodate the incoming ligand in an associative substitution reaction as illustrated in Scheme 2-4. This is mostly observed with indenyl complexes rather than Cp ligands. Even though Cp ligands are said to undergo “ring slippage” from η5 to η3, the process is energetically favoured by the gain in aromaticity of the six-member ring in the η3- intermediate for indenyl complexes, stabilization not available to Cp [5].

L -CO Rh Rh Rh slow fast CO OC CO OC OC L L η5 η5 η3

Scheme 2-4 Schematic representation of ring slippage to open a new co-ordination site [25]

In 1969, Hart-Davis and Mawby [26] found that the methyl migration in [Mo(η5-

C9H7)(CO)3Me] was ligand assisted. The SN2 nature of this reaction was attributed to the ability of the indenyl to undergo η5 – η3 migration allowing associative attack on the metal centre. They believe that the benzene portion of the indenyl ring provides stabilisation of the η3, since the cyclopentadienyl analogue reacts ten times slower. In another study, Mawby and Jones [27] showed that carbonyl substitution reactions of the indenyl complex [(η5-

25 Rerek M. E., Ji L. N., Basolo F., J. Chem. Soc., Chem. Commun., 1983, 1208. 26 Mawby R. J., Hart-Davis A. J., J. Am. Chem. Soc., 1969, 2403. 27 Mawby R. J., Jones D. J., Inorg. Chim. Acta., 1972, 6, 157.

24 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

C9H7)Fe(CO)2I] proceed by an SN1 mechanism and are faster than the analogous reactions of 5 the cyclopentadienyl complex [(η -C5H5)Fe(CO)2I] by a factor of about 600 at 95 ˚C. When a 5 more crowded tetrahydroindenyl complex, [(η -C9H11)Fe(CO)2I] was reacted, a lower rate compared to the Cp-complex was observed. They therefore suggested that the acceleration in reaction of indenyl complexes is due to the electronic nature of the indenyl ligand and not the steric factor.

According to Casey and O’Connor [28] the η3 intermediate has not been observed. In an 3 attempt to observe η intermediate in the reaction of [(C9H7)Re(CO)3] with PPh3, they observed dramatic effects of the indenyl group on the η5 to η1 conversion. The equilibrium between η5 and η1-cyclopentadienyl complexes lies much further toward the η1 complex for the indenyl system than for the cyclopentadienyl system. Six years later, [29] spectroscopic 3 3 evidence was presented for the formation of the η -indenyl intermediate [(η -C9H7)Fe(CO)3H] forming at low temperature prior to the formation of the normal hydride, [η3-(

C9H7)Fe(CO)2H].

Photodecarbonylation of the acetyl complex [CpFe(CO)2(COMe)] (where Cp is C5H5 or can be replaced by Ind for C9H7) suggested a ring slippage mechanism due to a large difference ‡ ‡ between the volume of activation, ∆VL and ∆Vm (The subscripts L and m refer to trapping of the intermediate by L and methyl migration respectively) [30]. In another study, it was shown that the intermediate Iind is about five – fold more reactive toward methyl migration and toward trapping by various ligands than the Cp analogue, ICp. Based on these findings the authors concluded that the difference was too small to support a ring slippage mechanism for either type of the reaction. They also concluded that the ring slippage intermediate does not play a major role in either the methyl migration or ligand substitution reactions [31].

2.4.2. Oxidative Addition

Oxidative addition / reductive elimination processes are central to an enormous collection of

28 Casey C. P., O’Connor J. M., Organometallics, 1985, 4, 384. 29 Ahmed H., Brown D. A., Fitzpatrick N. J., Glass W. K., J. Organomet. Chem., 1991, 418, C14. 30 Ryba D. W., van Eldik R., Ford P. C., Organometallics, 1993, 12, 104. 31 McFarlane K. L., Lee B., Fu W., van Eldik R., Ford P. C., Organometallics, 1998, 17, 1826.

25 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS synthetically useful organometallic reactions and occur because of the ability of transition metal to exist in several different oxidation states [17].

Oxidative addition reactions may involve a co-ordinatively unsaturated 16-electron metal complex or five–coordinate 18-electron species and take the general form illustrated by Eq. (2-10) [32].

n n+2 LmM + XY ⇋ LmM XY (2-10)

The reverse reaction is called reductive elimination. Whether the equilibrium lies to the reduced-metal or oxidised-metal side depends significantly on (i) the nature of the metal and the ligands, (ii) the nature of the incoming ligands as well as the type of new bonds formed and (iii) the medium in which the reaction is conducted. If XY contains a multiple bond, addition to the metal centre occurs without cleavage of the bond, forming two new bonds with the metal automatically in the cis position in a three-member ring. If the XY bond is broken in the process, the product may be an isomer or a mixture of isomers that is thermodynamically most favourable. Scheme 2-5 gives some of the possible isomers.

X L1 L4 M

L2 L3 Y

X L1 L4 L1 Y M M + XY and many others L2 L3 L2 L3 L4

X L1 Y M

L2 L4 L3

Scheme 2-5 Possible isomers obtained in oxidation addition reaction [4].

32 Jordan B. R., “Reaction Mechanisms of Organometallic Systems”, Oxford University Press, Inc., New York, 1991, Chapter 5.

26 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

In the research done by Blake and Kubota [33] it was shown that the solvent governs the stereochemistry of product formed by addition of hydrogen halides to trans- halobis(arylphosphine)carbonyliridium(I). They used NMR studies of the methylated phosphine, to show that five of the eight possible geometric isomers are formed

Oxidative addition reaction takes place by a variety of methods depending on the nature of the reacting species and the reaction medium [17]. These methods are briefly discussed below.

2.4.2.1. Concerted Addition

Concerted oxidative additions are best known for non-polar substrates, particularly the oxidative addition of H2 (central to catalytic hydrogenation) and oxidative addition into C-H bonds (hydrocarbon activation) [17]. The first step involves formation of a sigma bond with the incoming ligand. This is then followed by the oxidative part of the reaction in which the metal electrons are transferred to the σ*orbital of the incoming ligand as depicted by Scheme

2-6. Electron-transfer from the H2 σ-orbitals decreases H-H bonding and increase M-H bonding. Both of these interactions lead to H-H bond weakening and M-H bond formation, even though they accomplish these charge transfers in different directions [34], [5]. The reaction proceeds through a three-centre transition state and the product forms with new bonds formed cis to one another.

33 Blake D. M., Kubota., Inorg. Chem., 1970, 9, 4, 989. 34 Saillard J. Y., Hoffmann R., J. Am. Chem. Soc., 1989, 1067, 2006.

27 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

σ* σ σ* + - + Y + Z -

+ + CO L CO - M Y + Y Z L M L LnM X L Z - + Three-centre transition state X Z

Y L Z

X CO L

Scheme 2-6 The concerted oxidation reaction mechanism [4, 5]

2.4.2.2. SN2 Reactions

This type of oxidative addition is adopted for polarised AB substrates such as organic halides, RX, (R = -Me, -COMe, Phenyl and allyls, X = halides). A pair of electrons from the metal is used to break the A-B bond in the reagent. The attack of the metal electron is directed at the 2+ - - least electronegative atom to give LnM , A and B ions [6]. According to Roodt, et al. [35] more polar and better donor solvent molecules accelerate the reaction. They reasoned that the function of the solvent is to ease the charge separation during rearrangement and formation of the five co-ordinate intermediate, which is formed during an oxidative addition of methyl iodide on square planar complexes. A transition state involves either retention or inversion of configuration at the C atom with the latter being energetically disfavoured [5]. SN2 is enhanced by a nucleophilic metal, but steric hindrance slows the reaction down. The general idea is illustrated in Scheme 2-7 below.

35 Roodt A., Basson S. S., Venter J. A., Leipoldt J. G., Inorg. Chim. Acta, 1987, 128, 31.

28 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

R X

L1 L2 L L δ+ δ- L1 L2 M + R X δ+ δ+ δ- M M R X L3 L4 L3 L4 L L

Linear transition state

R R + L X L L possible M M isomerization X- L L L L L Ionic intermediate cis addition No isomerization

R L L

L L X Trans addition

Scheme 2-7 The SN2 oxidative addition mechanism [4].

2.4.2.3. Radical Mechanism

Two subtype radical processes are known (non-chain and chain). Non-chain is functional in the addition of a particular collection of alkyl halides, (RX), to Pt(PPh3)3 (when R = Me, Et then X = I or R = PhCH2 then X = Br). This involves a transfer of X from RX to the metal centre, leading to a one-electron oxidation of the metal as a result of the transfer. For that reason, two radicals are formed, which rapidly recombine to give the product before either can escape from the solvent cage. Eq. (2-18) to (2-20) give an example of a non-chain radical reaction.

29 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

fast PtL3 → PtL2 + L (2-11)

slow PtL2 + RX → •PtXL2 + R• (2-12)

fast •PtXL2 + R• → RPtXL2 (2-13)

2.4.2.4. Ionic Mechanism

For species that are known to ionise, (such as HBr, HCl, etc.) the ionic mechanism is most likely and will be especially favoured in more polar solvents [32]. Ionic compounds dissociate and the proton adds to the metal centre to give a co-ordinate intermediate. Intramolecular isomerization followed by co-ordination of the halide anion, X-, gives the final product.

+ - XY ⇋ X + Y (2-14)

- X+ LnM + Y → {LnM(Y)} → LnM(Y)(X) (2-15)

2.5. PROXIMITY INTERACTION

Once the substrates have been co-ordinated to the catalytic site, they have to interact with themselves or with an external substrate to give either a further activated intermediate or the product(s) of the catalytic cycle [23]. Interaction processes including carbonyl insertion are discussed briefly.

2.5.1. Migratory Insertion Reaction

The general reaction in Eq. (2-16) a 1,2 shift of a ligand L to an unsaturated fragment XY,

30 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS both co-ordinated to a metal M, is of obvious synthetic value [36].

' L L L L' M X Y M X Y (2-16)

If XY is carbon monoxide and L is an alkyl, the alkyl migration to the carbonyl is the key C-C bond formation pathway in catalytic carbonylations such as acetic acid synthesis from methanol and many others [17], [37], with CO as the major C1 building block of the chemical industry [38]. Carbonylation of alkyl(pentacarbonyl)manganese by carbonyl (Eq. (2-17)) is the classical example of a migratory insertion reaction [39]. Most studied systems follow the same pattern shown by this best-known case. Therefore the classic results will be used to explain the general procedure of the process.

CO CO CO CO CO O OC Mn CH3 OC Mn C OC OC CH3 CO CO (2-17)

At first sight, this reaction appears to be an insertion of the entering CO into the Mn-Me bond; therefore the name insertion has continued to be used. However, phosphines and other nucleophiles (L) bring about an analogous transformation as shown in Eq. (2-18)

CO CO CO CO Mn O OC CH3 + L OC Mn C OC OC CH3 CO L (2-18)

This could be an insertion of an already co-ordinated CO into the Mn-Me bond, but it might

36 Berke H., Hoffmann R., J. Am. Chem. Soc., 1978, 100, 23, 7224. 37 (a) Moss R. J., Anderson J. M., George R., J. Organomet. Chem., 1995, 505, 131, (b) Moss R. J., Anderson J. M., S.-Afr.TydSkr.Chem., 1997, 50(3), 144, (c) Chen J., Wu C., Huang B., Lin Y., Wang Y., J. Organomet. Chem., 1993, 454, 173. 38 Massick S. M., Büttner T., Ford P. C., Inorg. Chem., 2003, 42, 575. 39 Calderazzo F., Angew. Chem. Int. Ed. Engl., 1977, 16, 299.

31 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS also be migration of the methyl group to a co-ordinated CO. Stereochemical evidence [40], [41] shows that nucleophile insertion in the second step of this reaction appears cis to the newly formed acyl group. Therefore the process may be viewed as a migration of methyl group to an adjacent CO resulting in an intermediate (a formally co-ordinatively unsaturated species) followed by addition of labelled CO or any other nucleophile to the vacant coordination site of the intermediate.

The mechanistic conclusion for this system was drawn based on the product distribution from the reverse reaction (decarbonylation of CO) as shown in Scheme 2-8. The expected product distribution for methyl migration is 25 percent with labelled CO trans to the methyl group, 50 percent with labelled CO cis to the methyl group and 25 percent with no label. On the other hand, if the reaction goes by insertion of a co-ordinated CO, then the products should be 75 percent with labelled CO in the cis position and 25 percent with no label. The results of both studies give a product distribution consistent with methyl migration.

CO CO 25% OC* Mn CH3 OC CO CO CO CO CO CO CO *CO O OC Mn CH + - CO 50% 3 OC Mn C OC Mn CH OC 3 OC CH3 OC CO *CO *CO CO CO 25% OC Mn CH3 OC CO

Scheme 2-8 Schematic representation of product distribution during decarbonylation of Mn(CO)5(COMe) [40].

CO insertion into metal-carbon σ-bonds and related reactions are highly stereo specific, both

40 Noack K., Calderazzo F., J. Organomet. Chem., 1967, 10, 101. 41 Flood T. C., Jensen J. E., Statler J. A., J. Am. Chem. Soc., 1981, 103, 4410.

32 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS at carbon and at the metal.

2.5.2. Mechanism and Rate Law of the Reaction

Previous kinetic studies have indicated a mechanism in which alkyl migration from metal to CO occurs in the first step and an unsaturated 16 electron or solvent saturated acyl intermediate is generated. Attack of a nucleophile on the intermediate produces the final product [42], [43]. This is illustrated in Scheme 2-9 and Eq. (2-20).

A number of studies [42], [43] showed that solvent co-ordination enhances the rate of migratory reactions Marynick and Derecskei-Kovacs [44] however found that the solvent blocks the site destined for the incoming nucleophile and therefore would not enhance the rate of nucleophilic attack. On the other hand, Bassetti and co workers [45], [46], [47] proposed a rapid formation of an outer-sphere complex between the alkyl complex, [(CnHn)Fe(CO)2Me], and the phosphine ligand, followed by simultaneous co-ordination of the phosphine ligand and migration of the alkyl group. Scheme 2-9 and Eq. (2-21) best describe this set-up. Full derivations of Eq. (2-21 and 2-24) are given in the Appendix (A5) as Eq A5-8 and A5-20 respectively.

K1, k1 [(η5−CnHn)Fe(CO)2(Me)] + [PX3] [(η5−CnHn)Fe(CO)2(Me), PX3]# k-1 k 5 # 2 5 [(η −CnHn)Fe(CO)2(Me), PX3] [(η −CnHn)Fe(COMe)(CO)PX3]

5 Scheme 2-9 Proposed pathway that migratory insertion follows [46] here (η -CnHn) is either a Cp or indenyl ring.

42 Pearson R. G., Basolo F., Buttler I. S., Inorg. Chem., 1967, 6, 2074. 43 Bibler J. P., Wojcicki A., Inorg. Chem., 1966, 5, 889. 44 Marynick D. S., Deresckie-Kovacs A., J. Am. Chem. Soc., 2000, 122, 2078. 45 Bassetti M., Mannina L., Monti D., Organometallics, 1994, 13, 3293. 46 Bassetti M., Monti D., Organometallics, 1993, 115, 4658. 47 Bassetti M.,Allevi M., Lo Sterzo C., Monti D., J. Chem. Soc., Dalton Trans., 1996, 3527.

33 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

# 5 # Fe,L refers to the intermediate species, [(η -CnHn)Fe(CO)2(Me), PR3] and L is the tertiary phosphine ligand, PX3.

# Rate = k2[Fe,L ] (2-19)

[Fe,L#] represents the concentration of the activated species, [L] is the concentration of the phosphine ligand and k2 is the rate constant for methyl migratory step.

k K [L][Fe] k = 2 1 Tot (2-20) 1 + K1[L]

Here K1 is the equilibrium constant for the formation of the activated species and [Fe]Tot is the total amount of iron complex in the reaction mixture.

k2 K1[L] kobs = (2-21) 1 + K1[L]

kobs is the observed rate constant. The reverse reaction for second step, decarbonylation is assumed to be negligibly slow; therefore the value of k-2 is neglected in this case.

The expression in Eq. (2-21) varies depending on the conditions for instance when [L] >>

[Fe] then K1[L] >> 1 therefore Eq. (2-21) reduces to kobs = k2 with a plot of kobs against [L] being independent of phosphine concentration, [L], at higher phosphine concentrations. A linear plot is obtained if the inverse of the observed rate constant is plotted against [L]-1 giving a slope and the y intercept equal to the inverse of K1k2 and k2 respectively according to Eq. (2-22) below.

1 1 1 1 = + (2-22) k K k [L] k obs {1 2 {2 slope intercept

At low ligand concentration K1k2 is far smaller than one therefore it is neglected. As a result a plot of the observed rate constant against ligand concentration gives a straight line that passes through the origin with a slope equal to K1k2. Under such conditions there is little or no

34 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS accumulation of intermediate.

In the two situations explained above a limiting value would be reached for the plot of kobs against the concentration of the phosphine ligand. According to Eq. (2-24) the limiting value at high concentrations of the phosphine ligand would be equal to the value of k1 and it is independent of the phosphine ligand. On the other hand Eq. (2-21) gives the limiting value as k2 and it is expected to vary with varying phosphine ligands.

K1, k1 # [(η5−CnHn)Fe(CO) 2 (Me)] + S [(η5−CnHn)Fe(COMe)(CO)S] k-1

k2 [(η5−CnHn)Fe(COMe)(CO)]# + PX3 [(η5−CnHn)Fe(COMe)(CO)PX3] -S

5 Scheme 2-10 The proposed mechanism for migratory insertion [42], [48]. (η -CnHn) is either a Cp or an indenyl ring.

# 5 # Fe refers to the solvated intermediate species, [(η -CnHn)Fe(COMe)(CO)S] , L is the tertiary phosphine ligand, PX3

# Rate = k2[Fe ][L] (2-23)

[Fe#] is the concentration of the intermediate or transition state species, [L] is the concentration of the phosphine ligand and k2 is the rate constant for phosphine co-ordination.

Application of the steady state approximation followed by integration gives the observed rate constant (Eq. (2-24)).

k2 k1[L] kobs = (2-24) k-1 + k2 [L]

Here kobs is the observed rate (experimentally obtained rate constant), k1 is the rate constant

48 Mawby R. J., Basolo F., Pearson R. G., J. Am. Chem. Soc., 1964, 86, 3994.

35 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS for methyl migratory and k-1 is the rate constant for decarbonylation reaction.

If k-1 is comparable to k2[L] the intermediate reacts with L at a rate that is comparable to the reverse migration. The values of k1, k2 and k-1 are obtained from a plot of 1/kobs versus 1/[L]

(Eq (2-25)) where 1/k1 is the y-intercept and the slope is equal to the value of k-1 / (k1k2).

1 k 1 1 = −1 + (2-25) k k k [L] k obs {1 2 {1 slope intercept

According to Eq. (2-24) an excess of L means that k-1 is comparatively smaller than k2[L], therefore is negligible and as a result, kobs is practically identical to k1.

kobs = k1 (2-26)

That means that the overall reaction is governed by k1, indicating this is a first order reaction. At a very high concentration of L the rate becomes independent of [L] and a limiting rate is reached hence kinetic expression in Eq. (2-25) is used to determine the observed rate constant.

If k-1 is very large relative to k2[L] (i.e. [L]<<<) then Eq. (2-24) changes to Eq. (2-27).

k1k2 [L] kobs = (2-27) k-1

Under such conditions the intermediate almost always goes back to the starting material and the second step, attack by L, governs the overall rate, consequently the reaction is second order. A plot of the observed rate constant, (kobs), against the concentration of the ligand, [L], gives a straight line that passes through the origin with a slope equivalent to {(k1k2)/k-1}.

36 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

2.5.3. Factors Affecting Migratory Insertion

Like many other chemical processes migratory insertion is dependent on a variety of factors, some of which are discussed below.

2.5.3.1. Ligand Effects

Ligand effects are the changes that occur in the kinetics and thermodynamics of reactions when the electronic and steric properties of ligands or incipient ligands are altered [49]. They play a key role in determining organometallic reactivity trends and catalytic behaviour [50].

2.5.3.1.1. Steric Effects

Steric effects for monodentate phosphine ligands are quantified by Tolman’s cone angle [51] and are depicted in Figure 2-3 below. The size of tertiary phosphine ligands, PX3, can be adjusted by changing R. Appendix section A.3 shows the determination of the effective cone 5 angle for P(p-MePh)3 from the crystal structure determination of [(η -

C5H5)Fe(CO)(COMe){P(p-MePh)3}].

R R R P 2.28A θ M

Figure 2-3. The schematic representation of a Tolman cone angle [51].

49 Golovin M.N., Rahman M., Belmonte J. E., Giering W. P., Organometallics, 1985, 1981. 50 Gonsalvi L., Adams H., Gunley G. L., Ditzel E., Haynes A., J. Am. Chem. Soc., 1999, 121, 11233. 51 Tolman C. A., Chem. Rev., 1977, 77, 313.

37 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

5 A large decrease in the rate constant k2 is observed for [(η -C5H5)(CO)2Fe(CH2Cy)] systems when the cone angle of the tertiary phosphine that replaces the solvent molecule in the intermediate acyl exceeds a critical value. This effect is enhanced in the corresponding (pentamethylcyclopentadienyl)Molybdenumbenzyl complex and only relatively small ligands such as dimethylphenylphosphine react. It is believed that the substantial steric effects may largely be reflecting the influence of the large, non-labile cyclopentadienyl group [52].

Table 2-1 Observed rate constants for migratory carbonyl insertion reactions induced by different phosphine ligands showing the dependence of rate on steric properties of phosphine ligands. a) b) Ligand k2K1 Cone angle (Å) -5 PPh3 5.6 x 10 145 -5 PMePh2 7.0 x 10 136 -4 P(Me)2Ph 7.4 x 10 122 a) Ref. [43] b) Ref. [47]

k1 decreases with increasing effective size of the solvent in a series of α-substituted 5 tetrahydrofurans for the reaction of diphenylmethylphosphine with [(η -C5H5)(CO)3Mo(Me)]. This is thought to reflect increasing steric hindrance for the co-ordination of the tetrahydrofuran to molybdenum [53], [58].

2.5.3.1.2. Electronic Effects

Earlier studies [54] done on steric effects on migratory insertion reactions showed a rate enhancement with increase in the size of the alkyl group, R, in the k1 step of the reaction represented in scheme 2-8 (L is a tertiary phosphine and S is a solvent). In the study conducted by Bassetti and co-workers on the reaction between an alkyl complex,

[(C9H7)Fe(CO)R] and different phosphine ligands, the equilibrium constant (K) for the

52 Cotton J. D., Markwell R. D., Organometallics, 1985, 4, 937. 53 Wax J. M., Bergman R. G., J. Am. Chem. Soc., 1981, 103, 7028. 54 Anderson J. M., Moss R. J., J. Organomet Chem. 1995, 494, 105.

38 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS interaction between the phosphine ligand and the isopropyl complex (complex 1) increased -1 -1 -1 from PPh3 (K = 1.5 M ) to PPh2Me (K = 10 M ) to PPh(Me)2 (K = 24 M ). Smaller and opposite dependence on the nature of the phosphine was observed with the methyl complex (complex (2)). The conclusion they drew from this data is that association of the crowded isopropyl complex (1) with PPh2Me and PPh(Me)2 is greater than that of the methyl complex (2) and the interaction is not affected by the steric hindrance of the alkyl group. They therefore attributed the enhanced association to electronic effects, in particular the changes in the metal complex electron density caused by branching of the alkyl complex (1). The change in the electron density on the metal centre was manifested in the carbonyl stretching values they obtained for the two complexes. Lower stretching frequencies for the isopropyl complex (1) (2003.7 and 1950.2 cm-1) compared to 2011.7 and 1957.9 cm-1 for the methyl complex (2) indicated stronger π back-bonding from the metal to carbonyl in the isopropyl complex (1) than in the methyl complex (2).

Table 2-2 Dependence of the observed rate constant, kobs, on the chain length for the reaction between L 5 and [(η -C5H5)Fe(CO)2R] (L phosphine ligands and R is alkyl groups with different chain length) c) kobs pKa 1 a) 1 i a) 1 Ligand -Me -Et -Pr a) 2 -6 -5 -5 PPh3 3.5 x 10 1.09 x 10 3.16 x10 2.63 a) 2 -6 -5 -5 PPhMe2 7.1 x 10 1.62 x 10 4.17 x 10 6.5 -6 b) 3 -5 a) 2 -5 b) 4 PPh2Me 1.42 x 10 1.51 x 10 9.87 x 10 4.57 a) Ref. [55] b) Ref. [56] c) Ref. [51]: 1. -Me done at T= 59.0 ˚C, -Et at T = 47.5˚ and -Pri at T = 31.0˚C 2. [PR3] = 0.32-0.33 M 3. [PR3] = 0.130 M, T= 50.0 ˚C in THF 4. [PR3] = 0.131, T = 40.0 ˚C in THF

Chen, Wu, Huang, Lin, Wu and Wang [57] also observed this enhancement for a metal ethyl complex compared to the corresponding methyl complex. In addition they showed that metal carbonyl complexes with methyl ligands show reduced rates of insertion as additional

55 Green M., Westlake D. J., J. Chem. Soc. (A), Inorg. Phys. Theor., 1971, 367. 56 Bassetti M., Allevi M., Lo Sterzo C., Monti D., J. Chem. Soc., Dalton Trans., 1996, 3527. 57 Chen J., Wu C., Wu I., Huang B., Wang J., J. Organomet. Chem., 1993, 454, 173.

39 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS electron-withdrawing substituents are added. They claim that factors other than the chain length of an alkyl group make a substantial difference in the reaction rate. For example they observed a facile CO insertion under relatively mild conditions (25 ˚C, 40 psi CO) in a binuclear ruthenium-methylene complex [Cp(CO)2Ru]2(µ-CH2) in contrast to no CO insertion even at 100 ˚C and 1000 psi CO pressure in ruthenium methyl complex and its analogues

Cp(CO)2RuCH2X (X = OMe, OC(O)Me). The facile CO insertion reaction of the ruthenium- methylene complex was rationalised with a relative large Ru-C-Ru angle (123˚), which is larger than that of the sp3 hybrid orbital (109.5˚). These results suggest that in addition to electronic and steric effects, the bond angle of M-C-M may also have some effect on the rate of CO insertion reactions.

Most of the M-C-Si angles of complexes containing CH2Si(Me)3 groups are larger than that of hybrid orbitals. The following order of reactivity was determined for Cp(CO)2FeMe in DMSO.

S i i n ((Me)3Si)2CH >> (Me)3CCH2 > Bu > Pr > (Me)3SiCH2 > CyCH2 > Bu > Et > Pr > Me

It is clear that both steric and electronic effects are involved and the former is more important. According to Cotton and Kroes [58] this enhanced reactivity is due to the accelerating effect of the electron releasing group which makes it possible for the trimethylsilyl methyl complexes to undergo CO insertion reaction under mild conditions. It is noted that this group has resulted in geometrical strain at the carbon atom.

Migratory insertion is considerably favoured by monoelectronic oxidation to 17 electron radicals. Vlček [59] reported that it could also be catalysed by monoelectronic reduction to 19 electron radicals. Because of the ease at which interconversion of 17 to 19 electron radical can occur it is not surprising that migratory insertion may be accelerated either oxidatively or reductively. This is in agreement with the observation that was made by Magnuson et al. [60] that migratory insertion is very rapid on oxidation of the alkyl complex. They argued that

58 Cotton J. D., Kroes M. M., Markwell R. D., Miles E. A., J. Organomet. Chem., 1990, 388, 133. 59 Vlček A. A., Miholová D., J. Organomet. Chem., 1982, 240, 413. 60 Magnuson R. H., Zulu S., T’sai W., Giering W. P., J. Am. Chem. Soc., 1980, 102, 6888.

40 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS there is formation of short-lived organometallic radicals that are generated by one-electron oxidation of [Cp(CO)2FeMe] and [Cp(CO)(L)Fe(COMe)]. They describe these intermediates as acyl cation radicals. Similar observations was made by Prock et al. [61], who found that + iron(III) complex [n-Cp(CO)(PPh3)FeMe] undergo carbonylation at least one million times faster than the analogous iron(II) complex.

Another observation was that the reaction is most likely to be accompanied by solvent incorporation as illustrated by the equation below.

−e + fast,S + Cp(CO)2FeMe → [Cp(CO)2FeMe] → [Cp(CO)(S)FeCOMe] (2-27)

2.5.3.2. Nature of the Metal

The rate of methyl migration decreases as the group is ascended. This is partly because of the higher strength of the metal-carbon alkyl bond that gets stronger down the group, which has been correlated with an increase in the bonding overlaps between the two sigma orbital of the alkyl and the (p, d)-hybrid sigma MLn on the metal centre. Moss and Anderson [62] observed this decrease. They used different transition metals (Fe-Ru) in the same group, while keeping the rest of the ligands the same. A 103 drop-off in the rate was observed. When the same alkyl migration reaction was done for [CpOs(CO)2R] (R = Me or C4H9) compounds, there was no detectable reaction at all after four days for R = Me and six days R= C4H9. Thus, the rate must be at least 104 times slower for the analogous ruthenium compounds.

2.5.3.3. Lewis Acid

5 Lewis acids such as AlBr3 induce rapid alkyl migration in [(Me)Mn(CO)5], [(η - 5 C5H5)Fe(CO)2(Me)] and [(η -C5H5)Mo(CO)3(Me)], forming cyclic adducts as seen in Scheme

61 Prock A., Giering W. P., Greene J. E., Meirowitz R. E., Hoffman L. S., Woska D. C., Wilson M., Chang R., Chen J., Magnuson R. H., Eriks K., Organometallics, 1991, 10, 3479. 62 Anderson J., Moss J. R., George R., J. Organomet. Chem., 1995, 505, 131.

41 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

2-11. Metal acyls are more basic at oxygen than the corresponding carbonyls because of the formation of enolate as shown below.

Me Me

M C M C O O- (2-28)

By binding to the oxygen, Lewis acids stabilize the transition state, speed up the trapping by L and therefore accelerate the reaction [6], [63].

Fe 125 oC + L Fe 136 atm O OC CH3 OC C OC L CH3

(very fast) AlBr3

Fe Br 20 oC AlBr2 + L Fe OAlBr OC 0.5 atm 3 C O OC C L CH CH3 3

Scheme 2-11 The diagram of Lewis acid assisted Migratory insertion [64].

2.5.4. Reductive Elimination

Once the desired product has been formed it needs to dissociate from the catalyst regenerating it for the next cycle. One such process, which facilitates the dissociation of the products from the metal centre, is reductive elimination. Like oxidative addition, reductive elimination also

63 McLain S. J., J. Am. Chem. Soc., 1983, 105, 6355. 64 Butts B. S., Strauss H. S., Holt M. E., Stimson E. R., Alcock W. N., Shriver D. F., J. Am. Chem. Soc., 1980, 102, 5093.

42 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS takes place by means of different mechanisms. A variety of elimination reactions are analogous to concerted oxidative addition reactions and proceeds via a non-polar, non-radical three-centre transition state with retention of the stereochemistry at the carbon. The groups to be eliminated must occupy the cis position on the metal, or must rearrange to cis if they are trans [17]. Reductive elimination processes are exceptionally important for organic synthetic application, because it is the major way in which transition metals are used to make carbon- carbon and carbon-hydrogen bonds.

2.6. APPLICATION IN CATALYSIS

Organometallic homogeneous catalysis is applicable to a wide variety of industrially important processes. Some of them are listed together with the type of catalyst used.

Carbonylation of methanol in the Monsanto process producing acetic acid using - rhodium catalysts (e.g. [Rh(CO)2I2] ). Hydroformylation process is used in the production of aldehydes. It uses cobalt or rhodium based catalysts. Reppe process involves carbonylation of methanol to acetic acid and methyl acetate, and carbonylation of the latter to acetic anhydride [5].

A brief discussion of Hydroformylation and the Monsanto processes follows.

2.6.1. Hydroformylation

The first generation of hydroformylation processes was exclusively based on cobalt as the catalyst metal. In general, during hydroformylation olefins and synthesis gas (CO and H2) react to form an aldehyde containing one more C atom than the original olefin, with low valence cobalt or rhodium as the catalyst. This process has become an important industrial

43 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS process since Otto Roelen first introduced it in 1938 [65]. Branched or straight chain aldehydes are formed in this process. It is one of the early stages in the production of vitamin A, with a plant producing 600 tons per year operating since 1970 [66], [67] (The productivity may have gone up over the years).

The accepted mechanism for the reaction entails the dissociation of a CO ligand to form a catalytically active HCo(CO)3 species. The unsaturated 16-electron HCo(CO)3 species forms a π complex with the olefin. Migratory insertion into the Co-H bond with subsequent coordination of CO produces the alkyl derivatives of the catalyst (RCo(CO)4, where R is the alkyl). This is followed by formation of a 16-electron acyl complex, R(CO)Co(CO)3, through intramolecular migration of the alkyl group to a cis CO ligand. Finally, oxidative addition of hydrogen to the acyl complex, followed by reductive elimination of the aldehyde and the recovery of HCo(CO)3 completes the catalytic cycle.

The second generation processes used rhodium as the metal because rhodium gave much faster catalysis and their feedstock utilization is much better than that of cobalt catalyst. The schematic representation of the entire process is given in Scheme 2-12 showing two alternative routes that the reaction follows. The first route is as explained above. The productivity level of the process is summarised in Table 2-3.

65 Goh K. S., Marynick D. S., Organometallics, 2002, 21, 2262 (and references therein). 66 Hermann W. A., Cornils B., Angew. Chem. Int. Ed. Engl., 1997, 36, 1048. 67 Paust J., Pure Appl.Chem., 1991, 63, 1, 45.

44 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

H CO Ph3P 1 Rh CO Ph3P CO Ph3P H 2 Rh OC R CO PPh3 R

Ph3P H Rh

OC PPh3 H R Ph3P R Rh RCH2CH2CHO Reductive elimination Ph3P CO Ph3P H CO Rh H OC Ph3P C(O)CH2CH2R PPh3 Rh Olefin Ph P H Insertion 3 CO Oxidative addition Ph3P CH2CH2R H2 Rh OC CH2CH2R PPh3 Ph P C(O)CH2CH2R 3 Ph3P Rh Rh CO Ph3P CO CO Insertion Ph3P CO CO

Scheme 2-12 Schematic representation of the formation of an aldehyde in hydroformylation process using rhodium based catalyst [11].

Table 2-3 Estimated Production capacities of aldehydes by hydroformylation for 1993a) Capacity [1000 t] b) c) Region C3 C4 C5-13 >C13 ∑ 2-EH Europe (West) 25 1600 535 85 2245 870 Europe (East) 785 785 450 North America 75 970 450 270 1765 300 Latin America 120 55 175 150 Far East (inc. 1040 140 30 1210 650 Australia) Total 100 4515 1180 385 6180 2420 [%] 2 73 19 6 100 a) Ref. [11] b) Including isobutanal c) 2-EH = 2-ethylhexanol

45 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

2.6.2. Monsanto Process

This process is used to manufacture acetic acid by rhodium-catalysed carbonylation of methanol (Eq. (2-4)) and was developed by Monsanto in the late 1960s.

Cat.∆at∆T MeOH + CO → MeCOOH and/or (MeCO)2O (2-24)

It is one of the most important industrial processes responsible for over 90 percent of all new acetic acid capacity worldwide since 1970. The low-pressure reaction conditions, the high catalyst activity and exceptional product selectivity are key factors in the success of this process. The Monsanto process together with the BASF cobalt catalysed high-pressure methanol carbonylation process was developed to utilise carbon monoxide and methanol from synthesis gas as alternative raw material. This was in competition with the ethylene–based acetaldehyde oxidation and saturated hydrocarbons oxidation processes.

2.6.2.1. Monsanto Technology

The key elements of this carbonylation process is the ability of a metal complex to undergo oxidative addition with methyl halide, carbon monoxide (CO) insertion into the methyl metal bond and reductive elimination of the acetyl halide.

When rhodium metal is used to catalyse the process, a common catalytic pathway proposed - involves the nucleophilic attack of the active rhodium catalyst complex, [Rh(CO)2I2] , on - methyl iodide, (MeI), to form a methylrhodium(III) intermediate, [Rh(Me)(CO)2I3] . Rapid methyl migration in this complex generates the acylrhodium(III) intermediate, - - [Rh(MeCO)(CO)I3] , which reacts with CO to form [Rh(MeCO)(CO)2I3] . Subsequent reductive elimination gives acetyl iodide as a product and regenerates the rhodium(I) anion. The acetyl iodide further reacts with hydroxyl containing compounds giving the corresponding acetyl derivatives and HI.

MeCOI + HOR → MeCOOR + HI (2-25) R = H, Me or MeCOOH

46 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

The HI liberated then reacts with methane, methyl acetate or dimethyl ether to generate the methyl iodide promoter.

HI + MeOR →MeI + HOR (2-26)

The overall reaction is given in Scheme 2-13 below.

O CH3OH H3C C OH HI

H O 2 O

CH3I H3C C I CO - I Rh oxidative I CO reductive addition elimination

CO - CH - 3 I I CO CO CO Rh insertion Rh I CO I C O I I CH3

Scheme 2-13 Simplified schematic representation of the Monsanto process [6].

In the original process the rate of the reaction is highly dependent on the nature of the medium with protic solvents accelerating the reaction rate. Acetic acid/water is the preferred medium in the commercial process. High concentrations of water have a positive influence on the reaction rate and the stability of the catalyst. Even though substantial amounts of water seem to play a crucial role in maintaining catalyst activity, stability and achieving economically acceptable carbonylation rates, separation of the products from water is very costly for a limited amount of product.

The need for a system which would operate at low water concentration, yet still maintain optimum rate and catalyst stability, led to development of low reaction water technology known as acid optimisation (AO). Celanese Chemicals initiated this in the early 1980s. In this process the catalyst stability is improved by addition of inorganic iodide in high

47 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS concentration, allowing operation at optimum methyl acetate and low water concentration in the reactor. Carbonylation rates comparable to those realised previously at high water concentration are achieved, with increased catalyst stability and product selectivity. This allowed an increase in production from 270 x 103 metric tons per year in 1978 to 1200 x 103 metric tons acetic per year in 2001 (with increase of 200 x 103 metric tons per year in 2000 only) at the Celanese Clear lake facility only.

2.6.2.2. Function of the Iodide Salt

The function of the iodide salt (LiI being the preferred salt) is to prevent the rhodium carbonyl catalyst complex from precipitating as insoluble rhodium triiodide (RhI3). The iodide salt also promotes catalyst activity, but the key factor is methyl acetate (MeOAc) concentration. Increasing (MeOAc) concentration over a range (ca. 0-1 molar) affords an increase in the carbonylation rate by raising the proportion of total rhodium in catalyst solution as the active - species, [Rh(CO)2I2] .

2.6.2.3. BP Low-water Technology (Cativa Process)

Carbonylation in the Cativa process is based on Ir complexes as opposed to Rh in Celanese chemicals. It makes use of simple iodide salts (Zn, Cd, Hg, Ga and In) or carbonyl complexes (Re, Ru, Os or W) to achieve commercially viable high reaction rates at low reaction water conditions with the reaction rate being essentially independent of CO pressure. The Ir catalyst is more stable with fewer by-products forming. The active species is the iridium - carbonyl iodide, [Ir(CO)2I2] .

The mechanism of the reaction involves two cycles, a neutral and an anionic catalytic cycle. The carbonylation steps are similar to those of rhodium-catalysed systems, except that in this - case the rate-determining step is the formation of the acyl complex, [Ir(COMe)(CO)2I3] via - methyl migration of the methyliridium(III) intermediate, [Ir(Me)(CO)2I3] . This involves the elimination of the iodide and subsequent addition of CO.

48 CHAPTER 2 ORGANOMETALLIC CHEMISTRY IN CATALYSIS

The development of CativaTM Bp has converted three world scale acetic acid plants from the old Rh-Based high-water Monsanto technology to an Ir-based low-water process. This also led to a start up of a 500 X 103 metric ton per year acetic acid plant in 2000 in Malaysia.

Based on this information the crucial role that organometallic chemistry plays in catalysis is manifest. Chapter three will discuss some of the organometallic complexes, which are deemed important in this study.

49 3 SYNTHESIS AND CHARACTERIZATION OF IRON COMPLEXES

3.1. INTRODUCTION

In this chapter the synthesis and characterization of cyclopentadienyl iron complexes containing different phosphorus(III) ligands are described. Characterization techniques employed included IR, multinuclear NMR spectroscopy as well as X-ray crystallography. Since the use of X-ray crystallography is central to much of the work discussed, the theory involved will be treated in a fair amount of detail.

3.2. SYNTHESIS AND SPECTROSCOPIC CHARACTERIZATION OF THE COMPLEXES

3.2.1. General Considerations

IR spectra for the determination of CO stretching frequencies were recorded as KBr disks on -1 a Perkin Elmer 881 spectrometer in the range 600-4000 cm and in CHCl3 (using solution cells) between 1600 and 2100 cm-1 on a Hitachi 270-50 IR spectrometer. NMR spectra were recorded in CDCl3 on a Varian Gemini 2000 NMR 300 MHz spectrometer. The chemical shifts (δ) are expressed in ppm relative to the Varian NMR standards indicated in Table 3-1 below. Pentacarbonyliron(0), [Fe(CO)5], and dicyclopentadiene (DCP) were obtained from CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

Aldrich and were used as received. Organic solvents were dried and purified according to literature procedures [1] and the methods are summarized in Appendix A.6.

Table 3-1 The chemical shifts (δ) as expressed in ppm relative to the NMR standards Nucleus Solvent Relative δ (ppm) 1 H CDCl3 7.24 31 P 85% H3PO4 0.0 19 F SO3CF3 -75.21

5 3.2.2. Synthesis of [(η -C5H5Fe(CO)2)2)], [1]

Scheme 3-1 below summarizes the overall reaction sequence for the synthesis of [(η5-

C5H5)Fe(CO)(COMe){L}], where L is PPh3, P(p-MePh)3, P(p-FPh)3, PPhCy2 or P(p-

MeOPh)3.

CO O OC OC Reflux Fe CO + Fe Fe + 6CO + H OC 2 CO CO 2 Fe(CO)5 DCP O

Dimer, [1], (yield = 59%)

THF 2 Na/Hg rt

2 CH3I 2[(η5−C Η )Fe(CO) CH ] 2[(η5−C Η )Fe(CO) ]- + 2Na+ 5 5 2 3 rt 5 5 2 Alkyl, [2], (yield = ca. 50%) Monomer

CH3CN Reflux 2 L

2[(η5−C5Η5)Fe(COCH3)(CO){L}]

5 Scheme 3-1 The sequence for the synthesis of [(η -C5H5)Fe(CO)(COMe){L}], where L = tertiary aryl phosphine ligands.

1 Perrin D. D., Armaredo L. F., “Purification of Laboratory chemicals”, Butterworth-Heinemann Ltd, 1988, Oxford, Chapter 3.

51 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

The experimental procedure for the formation of the dimer, [1], was as follows:

(a) Initially, the dimer, [1] was prepared according to the procedure used by Piper and Wilkinson [2]. Small yields were obtained despite the long reaction time of more than forty hours of reflux. At first it was thought that the [Fe(CO)5] used had decomposed, but IR spectra at room temperature in chloroform (between 2200 – 1650 cm-1) showed the expected strong peak around 2050 cm-1 with two shoulders at about 2130 and 1970 cm-1. The literature [3] value of the strong peak is at 2020 cm-1. According to this procedure, re-crystallization in chloroform with slow evaporation at -70˚C was supposed to yield a crystalline product. However, this was not obtained. When the same solution was cooled at ca. -50 ˚C for several days no crystals were formed either. When the solvent was evaporated, the maroon powder obtained was the desired complex [1], containing small amounts of decomposed product in the form of a brownish powder.

-1 -1 IR (CHCl3): ν = 1778, (bridging CO), 1996, 1956 cm (terminal CO), (KBr): ν = 1764 cm (bridging CO), 1976 and 1938 (terminal CO)

(b) A mixture of [Fe(CO)5] (10 ml, 76 mmol.) and DCP (70 ml, 460 mmol.) was refluxed for eight hours. After cooling to room temperature, the maroon coloured solution was filtered and washed with hexane (6x, 20 ml). Purple crystals of the dimer [1] were obtained and re- crystallised in chloroform and hexane to give a maroon red powder with some impurities in the form of a brown powder. (Yield = 7.77 g, 59 %).

1 -1 H-NMR: δ = 4.76 (s, 10H, (C5H5-Fe)2). IR (CHCl3): ν = 1770 cm (bridging CO), 2009, 1953 cm-1 (terminal CO), (KBr): ν = 1759 cm-1 (bridging CO), 1985, 1942 cm-1 (Terminal CO). m.p. = 194 ˚C.

2 Piper T. S., Wilkinson G. J., J. Inorg. Nucl. Chem., 1955, 1, 165. 3 Sheline R. K., Pitzer K. S., J. Am. Chem. Soc., 1950, 72, 1107.

52 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

5 3.2.3. Synthesis of [(η -C5H5)Fe(CO)2Me], [2]

(a) The alkyl complex, [2], was originally prepared according to the procedure of Piper and Wilkinson [4]. However, this method was time consuming, produced low yields and involved prior preparation of 6% (percentage dependent on how fast the mercury is added, 3% produced when mercury is added fast) amalgam. The preparation of the amalgam was done at a temperature exceeding 200 ˚C. Freshly cut Na metal (0.33 g, 14 mmol.) under liquid paraffin was melted and mercury (14.26 ml) was added slowly to the molten sodium with stirring to form a hard amalgam. Further heating and crushing in a mortar produced finely divided lumps, which were then cooled and stored in an airtight bottle. The product was washed with benzene prior to use.

To the solution of dimer [1] (0.20 g, 0.6 mmol.) in THF (5 ml), a solution of amalgam (ca. 4.0 g, which was prepared as explained in the previous paragraph) in THF (5 ml) was added. After stirring the maroon mixture for 18 hours a brown mixture was obtained. MeI (1 ml, 16 mmol.) was added and the mixture was stirred for 2 hours at 35-45ºC. This was followed by evaporation of the solvent under a stream of nitrogen. Extraction of the amorphous product with a hexane and water mixture gave the pure product in very small amounts. (Yield ca. 5 %)

Because of low yields obtained, characterization was not reproducible and is omitted. This procedure was not used further. The following procedure gave complex [2] in higher yield and was therefore used for the synthesis.

(b) Sodium amalgam was prepared by an exothermic reaction between sodium metal (1.0 g, 43 mmol.) and mercury (15 ml) while stirring. After cooling the product to room temperature, dry THF (50 ml) was added, followed by addition of the dimer [1] (2.5 g, 7 mmol.). The mixture was stirred vigorously for 25 minutes. MeI (2 ml, 32 mmol) was added while stirring continued for another 20 minutes. The mercury was separated from the solvent layer and the solvent evaporated under vacuum. A cooling probe was inserted inside the flask

4 Piper T. S., Wilkinson G., J. Inorg. Nucl. Chem., 1956, 3, 104.

53 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION and the product sublimed from the residue at room temperature under vacuum onto the water- cooled probe, yielding a fatty yellow crystalline compound [2] with a camphor smell. The product is air sensitive and changes colour from yellow to brown with time. (Yield = 1.35 g, ca. 50 %).

1 -1 H-NMR: δ = 4.73 (s, 5H, (C5H5-Fe), 0.14 (s, 3H, Fe-Me). IR (CHCl3): ν = 2006, 1946 cm (CO), (KBr): ν = 2006, 1946 cm-1. m.p. = 60-67 ˚C.

5 3.2.4. Synthesis of [(η -C5H5)Fe(CO)(COMe){PPh3}]

A mixture of PPh3 (0.13 g, 0.5 mmol) and the alkyl complex [2] (0.10 g, 0.5 mmol.) was refluxed for four days in MeCN (4 ml). IR was used to follow the reaction to completeness. More MeCN (2 ml) was added to increase the reaction volume before the reaction mixture was filtered, evaporated to dryness and re-dissolved in pentane. The product crystallized as a mixture of light and dark orange crystals. A few crystals of different colours were picked into separate containers and analysed separately before they were purified by column chromatography packed with silica gel using hexane/acetone (9:1) as eluent. This yielded only a dark orange crystalline product. (Yield = 0.15 g = 62 % containing 38 % oxidised

PPh3).

1 Light orange crystals analysis: H-NMR: δ = 4.40 (s, 5H, (C5H5-Fe), 2.30 (s, 3H, Fe-COMe), 31 7.35-7.66 (m, 15H, PPh3 and 15H, OPPh3) P-NMR: δ = 76.24 (s, 1P, PPh3), 29.90 (s, 1P,

OPPh3).

1 Dark orange crystals: H-NMR: δ = 4.40 (s, 5H, (C5H5-Fe), 2.30 (s, 3H, Fe-COMe), 7.35 - 31 7.80 (m, 15H, PPh3, 15H, OPPh3), P-NMR: δ = 76.24 (s, 1P, PPh3) 29.8 (s, 1P, OPPh3). IR (KBr): ν = 1921, 1604 cm-1. m.p. = 120-127 ˚C.

54 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

5 Table 3-2 Infrared data for [(η -C5H5)Fe(CO)(COMe){PPh3}] in different solvents at room temperature.

Solvent νCO (bridging CO) νCO (terminal CO) cm-1 cm-1

CHCl3 - 1916 Toluene 1608 1915 Hexane 1614 1923 DCM 1603 1915

The difference in colour of the two types of crystals mentioned above may be ascribed to the fact that the product contains a chiral centre, making it possible to have two isomers as depicted in Figure 3-1. NMR analysis of the separated crystals showed identical values.

Fe O O Fe CO OC P P Ph2 Ph2

(R)-(-) S-(+)

5 Figure 3-1 The two enantiomers [5] +/-[(η -C5H5)Fe(CO)(COMe){PPh3}]. Priority of ligand sequence used is Cp>PPh3>CO>COMe.

The only way to confirm this was with a crystallographic study, which was not deemed necessary for our aim.

5 Davies G. S., Dordor-Hedgecock M. I., Sutton K. H., Walker C. J., Bourne C., Jones R. H., Prout K., J. Chem. Soc., Chem. Commun., 1986, 607.

55 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

5 3.2.5. Synthesis of [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}], A

A mixture of P(p-MePh)3 (0.15 g, 0.5 mmol.) and the alkyl complex [2] (0.10 g, 0.5 mmol.) in MeCN (4 ml) was refluxed for ca. three days. IR was used to follow the reaction to completeness, after which the reaction mixture was cooled and filtered. This was followed by evaporation of the solvent and re-crystallization from MeCN. The product was obtained as yellow crystalline clusters that were purified on silica gel with hexane/acetone (9:1) as eluent.

(Yield ≈ 90 %, 0.23 g containing 7.7 % OP(p- MePh)3)

1 H-NMR: δ = 4.38 (s, 5H, C5H5-Fe), 2.30 (s, 3H, Fe-COMe), 2.33 (s, 9H, P(p-(MePh)3), 9H,

OP(p-MePh)3), 7.13-7.16 (d, 6H, P(p-MePh)3), 6H, OP(p-MePh)3), 7.30-7.38 (d, 6H, P(p- 31 MePh)3)), P-NMR: δ =73.68 (s, 1P, Fe-P(p- MePh)3), 29.67 (s, 1P, OP(p- MePh)3). IR (KBr): ν = 1898, 1597 cm-1. m.p. = 155-156 ˚C.

Table 3-3 Infrared data for [B] in different solvents at room temperature

Solvent νCO (bridging CO) νCO (terminal CO) cm-1 cm-1

CHCl3 - 1915 Hexane 1613 1923 DCM 1601 1913

5 3.2.6. Synthesis of [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}], B

A mixture of P(p-FPh)3 (0.16 g, 0.5 mmol.) and the alkyl complex [2] (0.10 g, 0.5 mmol.) in MeCN (4 ml) was refluxed. When the reaction was complete as ascertained by IR, (ca. 2 days) the solvent was evaporated and the residue dissolved in acetone and pentane (0.5:9.5).

The product was purified by column chromatography on neutral alumina (Al2O3) using hexane as eluent. It was obtained as yellow crystals in fair yields (Yield = 0.14g, > 50 % containing 19 % oxidised P(p-FPh)3).

56 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

1 H-NMR: δ = 4.39 (s, 5H, C5H5-Fe), 2.32 (s, 3H, Fe-COMe), 7.05-7.68 (m, 12H, P(p-FPh)3) 31 and 12H, (OP(p-FPh)3)), P-NMR: δ = 75.40 (s, 1P, Fe-P(p-FPh)3), 27.50 (s, 1P, OP(p- 19 FPh)), F-NMR: δ = -112.30 (s, 3F, Fe-P(p-FPh)3), -108.22 (s, 3F, OP(p-FPh)3). IR (KBr): ν = 1904, 1607 cm-1. m.p. = 149-151 ˚C.

Table 3-4 Infrared data for A in different solvents at room temperature.

Solvent νCO (bridging CO) νCO (terminal CO) cm-1 cm-1

CHCl3 - 1918 Hexane 1591 1926 DCM 1591 1917

5 3.2.7. Synthesis of [(η -C5H5)Fe(CO)(COMe){PPhCy2}]

A mixture of PPhCy2 (0.15 g, 0.5 mmol.) and the alkyl complex [2] (0.10 g, 0.5 mmol.) in MeCN (4 ml) were refluxed for six days. IR did not show any change, but purification with column chromatography packed with silica gel using hexane/acetone (9:1) as eluent produced a small amount of product. (16 mg =10 % yield).

1 H-NMR: δ = 4.39 (s, 5H, (C5H5-Fe), 2.67 (s, 3H, Fe-COMe), 7.39-7.90 (m, 5H, PPhCy), 0.85-2.18 (m, 22H, PPhCy), 31P-NMR: δ =72.27 (s, 1P, PPhCy). IR (KBr): ν = 1912, 1595 cm-1.

5 Table 3-5 Infrared data for [(η -C5H5)Fe(CO)(COMe){PPhCy2}] in different solvents

Solvent νCO (bridging CO) νCO (terminal CO) cm-1 cm-1

CHCl3 - 1912 Toluene 1603 1909 Hexane 1609 1917 DCM 1603 1909

57 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

5 3.2.8. Synthesis of [(η -C5H5)Fe(CO)(COMe){P(p-MeOPh)3}]

A mixture of P(p-MeOPh)3 (0.31 g, 1 mol) and the alkyl complex [2] (0.10 g, 0.5 mmol.) in MeCN (4 ml) were refluxed and IR was used to follow the reaction to completeness. A sticky black substance was obtained after evaporating the solvent, which was dissolved in hexane/acetone (9:1) and the product separated by column chromatography with minute yields. The product partially dissolved in the solvents used becoming more tarrish. This made it difficult to separate the product quantitatively from the impurities.

1 H-NMR: δ = 4.41 (s, 5H, (C5H5-Fe), 2.32 (s, 3H, Fe-COMe), 1.26 (s, 9H, P(p-MeOPh)3), 31 6.86-7.43 (m, 12H, P(p-MeOPh)3)), P-NMR: δ = 71.34 (s, 1P, Fe-P(p-MeOPh)3), 29.39 (s, -1 1P, OP(p- MeOPh)3). IR (KBr): ν = 1916, 1595 cm . m.p. = 160-163 ˚C.

5 Table 3-6 Infrared data for [(η -C5H5)Fe(CO)(COMe){P(p-MeOPh)3}] in different solvents

Solvent νCO (bridging CO) νCO (terminal CO) cm-1 cm-1

CHCl3 - 1912 Toluene 1603 1909 Hexane Insoluble - DCM 1603 1915

In addition to the techniques employed above to characterize the complexes, X-ray crystallography was also used.

3.3. CHARACTERIZATION OF SELECTED COMPLEXES BY X-RAY CRYSTALLOGRAPHY

As indicated in the introduction, X-ray crystallography forms an important part of this study, making a thorough understanding of the theory behind this technique imperative.

58 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

3.3.1. Theoretical Aspects

The study of diffraction patterns produced by X-rays on crystalline material, in particular single crystals, makes it possible to determine the size and the shape of the relevant unit cell and coordinates of the atoms within the unit cell [6]. Crystal structure determination is based on a simple physical principle: just as light is diffracted by a pair of slits, and the resulting interference pattern depend on the wavelength of the light and the distance between the slits, X-rays are also diffracted by pairs of atoms, and the interference pattern which is produced depends on the wavelength of the X-rays and distance between the atoms [7]. X-ray diffraction studies have proved to be the most powerful tool for the study of the internal structure of crystals [6]. Why is the internal structure of a crystal so important and how is the diffracted X-rays derived into valuable information? These two questions will be discussed fully in the sections to follow, beginning with the basic concepts of crystallography.

3.3.1.1. Basic Concepts in Crystallography

A crystal is a solid compound in which the atoms are periodically arranged in three dimensions. This geometric arrangement of the atoms is called a crystal structure. In a crystal structure there are also points with exactly the same surroundings, and the collection of these identical points is called a lattice. A set of vectors can be used to describe the periodicity of a structure. Vectors can also be used to repeat a point to reproduce a lattice. In three dimensions three vectors describe a lattice and these vectors define a volume, i.e., a unit cell [8]. Equation 3-1 below describes the volume of a unit cell where ρ is the density of a crystal, M is the molecular or formula weight, Z is the number of molecules or formula units, V is the volume and N is Avogadro’s number [8].

6 Mckie D., Mckie C., “Essentials of crystallography”, Blackwell Scientific Publication, Oxford, 1986, Chapters 6, 9. 7 Ebsworth E. A. V., Rankin W. H. D., Cradock S., “ Structural methods in inorganic chemistry”, Blackwell Scientific Publication, Oxford, 1987, Chapter 8. 8 Oskarsson Ǻ., “Series of Lectures”, 2001.

59 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

(MZ) ρ = (3-1) (NV )

Density is one of the physical properties of crystals that can be obtained early in a crystallographic investigation and can assist in determining the accurate value for the total molecular weight of a crystal using the above-mentioned equation [9]. Knowing the crystal structure provides geometric parameters of utmost importance in crystallography, such as bond angles, bond distances, etc. [7].

A crystal structure may be described by symmetry operations other than pure translation symmetry [8]. For symmetry operations to occur there should be a symmetry element that can be a point, axis or plane around which a symmetry operation is performed. The symmetry element determines the crystal system, and the shape is described by one of the 32-point groups (which represent one of the possible unique combinations of crystallographic symmetry elements). A lattice is described by one of the 14 lattice types and its structure by one of the 230-space groups (which describes the only way in which identical objects can be arranged in an infinite lattice) [8].

If a crystal is to be satisfactory for collecting X-ray diffraction data, two main requirements must be met [9]: It must possess uniform internal structure. It must be of proper size and shape. The pattern of reflections recorded will depend on the orientation of the crystal with respect to the incident beam. In view of the fact that most crystals contain many different atoms and the patterns for the various types of atoms do not exactly coincide; the diffracted beams are not in phase. It follows that the total intensity of a reflection depends on the position of the atoms in the cell [7]. However, it is the structure factors that are compared, not the intensities [8]. The structure factor, Fhkl, is the measure of amplitude of the reflections from the set of planes h, k, and l. It is therefore a function of the reflection indices and the position of the atoms in the

9 Stout G. H., Jensen L H., “X-ray structure determination, a practical guide”, 2nd Ed, John Wiley and Sons, New York, 1989, Chapter 4.

60 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION unit cell [7]. It is related to the experimentally observed intensities as shown in Eq. (3-2) and (3-3) [9].

| F | α I (3-2)

1  KI  2  (hkl)  | F |=   (3-3)  Lp 

K is a scale factor and Lp denotes Lorentz and polarization factors.

3.3.1.2. Structure Factor Fhkl

The electrons in atoms contribute to the scattering of X-rays; therefore the scattering amplitude of an atom is the sum of the contributions of all the electrons in all the atoms in the crystal. First, the scattering amplitude of a single electron and the variation in scattering amplitude with angle is determined. Then, the scattering amplitude of an atom is determined by summation of the contributions from all N electrons. (N = atomic number of the atom), taking into account the phase differences between the entire N scattered waves. A simple number, f, which is the atomic scattering factor, is defined as follows: (Eq. (3-4)) [10].

ω f = Atom (3-4) ωElectron

ωAtom is the amplitude scattered by an atom, ωElectron is the amplitude scattered by a single electron.

Therefore, the scattering amplitude of a unit cell is determined by summing the scattering amplitudes, f, from all the atoms in the unit cell, to all the atoms in the motif. The summation

10 Christopher H. C., “The basis of crystallography and diffraction”, 3rd Ed, Oxford University Press Inc, New York, 2000, Chapter 9.

61 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION takes into account the phase difference between all the scattered waves and a dimensionless number, Fhkl, the structure factor, expresses it as shown in Eq. (3-5) below [10].

ωUnit cell Fhkl = (3-5) ωElectron

ωUnit cell is the amplitude scattered by atoms in a unit cell, ωElectron is the amplitude scattered by a single electron.

Since Fhkl is expressed by both the amplitude of the scattering from a reflecting plane and the phase angle of the scattering wave, it is not a simple number like the atomic scattering factor, f. It is represented as a vector f or mathematically as a complex number and is resolved into its real and imaginary components (f = fcosα + ifsinα) [10], [8]. This vector makes an angle α with the origin. This angle is said to be its phase and it is described by Eq. (3-6) [8].

αn = 2π(hxn + kyn + lzn ) (3-6)

Joseph Fourier has shown that periodic function can be described as the sum of simple sine and cosine functions (i.e. sum of exponential terms, cos α + sin α = eiα ) therefore vector f is expressed as seen in Eq. (3-7) [8].

f cos α + i f sin α = f eiα (3-7)

The contribution of the nth atom to the structure is then in exponential form as illustrated by Eq. (3-8) that follows [8].

iαn 2πi(hxn +kyn +lzn ) f n e = f n e (3-8)

The resultant of the N waves, Fhkl is given by Eq. (3-9) below [8].

2πi(hxn +kyn +lzn ) F(hkl) = Σf n e (3-9)

62 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

As mentioned before, the scattering of X-rays by a crystal is almost entirely due to the interaction between X-rays and electrons. Therefore, the density of the scattering matter at a point x, y, z in the unit cell can be equated with the electron density (number of electrons / volume) at that point. It is possible to calculate the structure factor using the electron density (number of electrons / volume) [8].

Vρ(x, y, z) dx dy dz (3-10)

The structure factor then takes the form illustrated in Eq. (3-11) [8].

2πi(hx + ky + lz) F(hkl) = ∫Vρ(x, y, z) ℮ dV (3-11)

3.3.1.3. Fourier Synthesis

A function F(s) is known as the Fourier transform of ƒ(x) and the two functions are related as illustrated below in Eq (3-12) and (3-13) [8], [6].

F(s) = ∫ f(x)℮2πi(hx + ky + lz)dx (3-12)

f(x) = ∫ F(s) ℮-2πi(hx + ky + lz)ds (3-13)

When this theory is applied to the structure factor expression, Eq. (3-11), the electron density expression is changed to Eq. (3-14) below [8].

-1 -2πi(hx + ky + lz) ρ(x, y, z) = V ∫ F(hkl) ℮ dV (3-14)

It is therefore possible to calculate the electron density distribution over the cell and to locate the position of the atoms at a point with highest electron density, provided the accurate values of structure factor and phase (αhkl) of a diffracted beam are available. The diffraction pattern lacks information about the phase and this phase problem can be solved by various methods such as the Patterson method [7].

63 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

3.3.1.4. Patterson Maps

In 1935 Patterson postulated that if electron density at a point (x, y, z) is multiplied by the electron density at (x + u, y + v, z + w) and then integrated over the unit cell volume and finally multiplied by unit cell volume, then a maximum peak will be observed only when there is electron density at both points (x, y, z) and (x + u, y + v, z + w) for all interatomic vectors [8]. Interatomic vectors are represented by Patterson maps. These maps are mostly functional if one atom is much heavier than the others so that the vectors involving the atom stand out clearly [7]. The heavier atom is called the phase determining atom. The whole postulate is indicated in Eq. (3-15) [8].

P(u, v, w) = V ∫ ∫ ∫ ρ(x, y, z) ρ(x + u, y + v, z + w)dxdydz (3-15)

2 The Fourier synthesis using the phase less quantity |Fhkl| gives the Patterson synthesis as shown in Eq. (3-16) [8].

-1 2 -2πi(hu + kv + lw) P(u, v, w) = V ΣΣΣ |F(hkl)| ℮ (3-16)

3.3.1.5. Least Squares Refinement

Once the heavy atom(s) has been located the structure factor (Fc) based on these positions can be calculated. This is called the trial structure. If the trial structure is substantially correct, there will be quite good agreement between the structure factors calculated from the trial structure and the measured structure factors. The quality of agreement is measured by the reliability index, R factor, defined as Eq. (3-17) illustrates [6].

Σ || F | - | F || R = o (hkl) c(hkl) (3-17) Σ | Fo(hkl) |

For experimental data of high quality and an accurately determined crystal structure, R would be expected to be no greater than 5 %. When the structure determination has reached a stage at which the R factor is already quite small, it becomes necessary in order to improve the fit

64 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION between observed and the calculated structure factors to take into account the anisotropy of atomic thermal vibrations. Least square method is used to refine the structure. This is the automatic procedure, which minimizes R1 or R2 by adjustment of the parameters of the trial structure. The function that is minimized is either one depicted by Eq. (3-18) or (3-19) below [6].

2 R1 = Σw(|sFo(hkl)| - |Fc(hkl)|) (3-18)

1 2 2 2 R2 = Σw (|sFo(hkl)| - |Fc(hkl)| ) (3-19)

3.3.1.6. Difference Synthesis

Since the coefficients used in Eq. (3-14) are observed structure factors combined with phases calculated from the trial structure, the electron map would be an image of the structure, which should be a closer approximation of the real structure. The electron density for the trial structure is given by Eq. (3-20).

-1 2 iα -2πi(hx + ky + lz) ρc = V ΣΣΣ|Fc| (℮ c )℮ (3-20)

Eq. (3-21) gives the electron density for the complete structure.

-1 2 iα -2πi(hx + ky + lz) ρo = V ΣΣΣ|Fo| (℮ 0 )℮ (3-21)

The difference between the real structure and the trial structure is given by ∆ρ(x, y, z) defined by the equation below [6].

-1 iαc −2πi(hx + ky + lz) ∆ρ(x, y, z) = V ΣΣΣ(| Fo(hkl) | − | Fc(hkl) |)(e )e (3-22)

The method that was employed to collect the crystallographic data and the results of refinement are explained in the next section.

65 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

5 3.3.2. Structure Determination of [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}], 5 A and [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}], B.

The crystals of the two complexes studied in this investigation were prepared as described in section 3.2.5 and 3.2.6. Suitable crystals were grown from pentane by slow evaporation of the solvent at room temperature.

3.3.2.1. General

The software package SHELXS/SHELXL-97 [11] was used for structure solutions and refinement. Molecular graphics were prepared using Diamond [12] for Windows. The structures were solved by standard Patterson and Fourier techniques and refined by full matrix least-square methods based on F2.

The positional and anisotropic parameters for all non-H were refined starting with isotropic thermal parameters before switching to the anisotropic thermal parameters.

3.3.2.2. Data Collection

The X-ray data sets were collected on the Bruker SMART CCD 1K area detector system using Mo radiation (λ = 0.71073 Å). The CCD area detector was mounted 4.5 cm from the crystal and the data set was collected by use of omega scans at room temperature. A graphite monochromator was used and the selected crystal was mounted on glass fibre with epoxy cement. X-rays were generated using a fine-focus sealed tube and an X-ray generator operating at 50 kV and 30 mA using an exposure time of 10 s/frame. A total of 1315 frames were collected using a frame width of 0.3˚ covering up to θ = 28.42˚ and 28.33˚ with 5 completeness of 96.6 % and 97.0 % being achieved for [(η -C5H5)Fe(CO)(COMe){P(p- 5 MePh)3}], A, and [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}], B, respectively. Each set of

11 Sheldrick G. M., SHELX97, “Program for crystal refinement”, University of Göttengen, Germany, 1997. 12 Brandenburg K., ”Diamond version 2.1a, Crystal Impact GbR, Bonn, Germany.

66 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION frames amounting to the total had a different angle for the crystal, but the first 50 frames were recollected at the end of data collection to check for any decomposition since the first collection. It was found that both crystals were stable during the data collection. No absorption correction was applied. Hydrogen atoms were placed in geometrically idealized positions and were refined using a riding model.

3.3.2.3. Results of Refinement

Details of the data collection and refinement are listed in Table 3-7. Figure 3-2 and Figure 3-4 below presents the molecular structures of the complexes studied and give the numbering of atoms. Both A and B crystallizes in the triclininic crystal system in the P1 space group with two molecules in the unit cell. The packing in the unit cell of the two complexes is presented in Figure 3-3 and Figure 3-5. Bond lengths and angles of interest in this study are tabulated below in Table 3-8 and 3-10. The coordinates and thermal parameters of the atoms are given in Appendix in Table A.1-1 and A.2-1 for A and B respectively, hydrogen coordinates and anisotropic displacement parameters are given in Table A.1-2 and A.1-3 and A.2-2 and A.2-3 respectively for A and B.

67 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

Table 3-7 Crystal data and structure refinement for A and B

A B

Empirical formula C29H29FeO2P C26H20F3FeO2P Formula weight 496.34 508.24 Temperature 293(2) K 293(2) K Wavelength (Å) 0.71073 0.71073 Crystal system, space group Triclinic, P1 Triclinic, P1 Cell dimensions a = 8.256(3) Å a = 8.2497(16) Å b = 9.552(3) Å b = 8.7164(17) Å c = 17.245(6) Å c = 16.655(3) Å α = 106.063(6) ˚ α = 80.074(4) ˚ β = 91.285(9) ˚ β = 78.618(4) ˚ γ = 98.721(6) ˚ γ = 86.783(3) ˚ Volume (Å3) 1288.9(7) 1156.2(4) Z, Calculated density 2, 1.279 mg/m3 2, 1.460 mg/m3 Abs coeff (nm-1) 0.670 0.766 F(000) 520 520 Crystal size (mm) 0.46 x 0.22 x 0.16 0.38 x 0.20 x 0.18 Theta range for data collection 1.23 to 28.33 ˚. 1.26 to 28.42 ˚. Index ranges -10 ≤ h ≤ 10, -10 ≤ h ≤ 10, -12 ≤ k ≤ 10, -11 ≤ k ≤ 11, -22 ≤ l ≤ 21 -22 ≤ l ≤ 14 Reflections collected 9083 8210 Unique reflections 6222 [R(int) = 0.0229] 5624 [R(int) = 0.0408] Completeness to 2theta 28.33, 97.00% 28.42, 96.60% Absorption correction None None Refinement method Full-matrix least-squares on F2 Full-matrix least-squares on F2 Data / restraints / parameters 6222 / 0 / 304 5624 / 0 / 301 Goodness-of-fit on F2 0.828 0.879 Final R indices [I>2sigma(I)] R1 = 0.0398, wR2 = 0.1147 R1 = 0.0429, wR2 = 0.1225 R indices (all data) R1 = 0.0692, wR2 = 0.1334 R1 = 0.0686, wR2 = 0.1394

68 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

5 3.3.2.4. The Crystal Structure of [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}], Acetyl(carbonyl)(η5-cyclopentadienyl)tri(p-tolylphosphine)iron-(II)

The molecular structure of A is presented in Figure 3-2 below.

Figure 3-2 Diamond [12] structure of the molecular structure of A showing numbering of atoms. The first digit gives the ring number, the second digit gives the number of the atom in the ring. Thermal ellipsoids are drawn at 40% probability level.

The coordination sphere of the iron atom can be best described by comparing it to a three legged piano stool, with the Cp ring being the seat and the carbonyl, acetyl and phosphine the three legs. The angles between the ligands representing the legs are almost 90 ˚, typical of octahedral coordination, with the Cp ring occupying the other three coordination sites of the octahedron. The distance of Cp-centroid to the iron is 1.751(7) Å. The CO ligand is essentially linear with the Fe-C(1)-O(1) angle of 174.3(3) ˚, with the Fe-P bond dist ance of

69 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

2.193(8) Å. The effective cone angle [13] is 152.6 ˚ compared to a Tolman [14] cone angle of 145 ˚. Calculation of the effective cone angle is shown in section A.3 of the Appendix. The torsion angle of the acetyl (C(1)-Fe-C(2)-C(3)) is –13.28(2) ˚. Table 3-8 below presents the bond distances and angles of importance for A.

Table 3-8 Selected bond lengths and angles for A

Distances [Å]

Fe-C(1) 1.746(3) C(44)-C(45) 1.393(4) Fe-C(2) 1.953(3) C(44)-C(43) 1.379(4) Fe-C(42) 2.106(3) C(41)-C(45) 1.386(5) Fe-C(44) 2.110(3) C(41)-C(42) 1.404(5) Fe-C(41) 2.111(3) C(42)-C(43) 1.394(4) Fe-C(45) 2.113(3) P-C(11) 1.833(2) Fe-C(43) 2.126(3) P-C(21) 1.836(2) Fe-P 2.193(8) P-C(31) 1.841(2) C(1)-O(1) 1.149(3) C(14)-C(17) 1.513(4) C(2)-O(2) 1.202(3) C(24)-C(27) 1.528(4) C(2)-C(3) 1.535(4) C(34)-C(37) 1.522(4) Fe-centroid 1.751(7) C(43)-centroid 1.182(4) C(41)-centroid 1.188(6) C(44)-centroid 1.179(7) C(42)-centroid 1.186(4) C(45)-centroid 1.181(1)

Angles [˚]

C(1)-Fe-C(2) 94.70(12) C(1)-Fe-centroid 124.21(2) C(1)-Fe-P 93.04(8) C(2)-Fe-centroid 120.56(2) C(2)-Fe-P 89.00(8) P-Fe-Centroid 126.05(1) C(45)-Fe-P 94.01(9) C(11)-P-Fe 116.92(7) O(1)-C(1)-Fe 174.3(3) C(21)-P-Fe 114.12(7) O(2)-C(2)-C(3) 115.2(3) C(31)-P-Fe 116.43(8) O(2)-C(2)-Fe 124.7(2) C(13)-C(14)-C(17) 121.5(3) C(3)-C(2)-Fe 120.0(2)

Torsion angle [˚]

C(1)-Fe-C(2)-O(2) 170.23(2) C(12)-C(11)-P-Fe 67.89(2) C(1)-Fe-C(2)-C(3) -13.28(2) C(32)-C(31)-P-Fe 163.64(2) C(22)-C(21)-P-Fe 57.70(2)

13 Roodt A., Otto S., Smith J., Inorg. Chim. Acta., 2000, 303, 295. 14 Tolman C. A., Chem. Rev., 1977, 77, 3, 313.

70 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

Table 3-9 below presents dihedral angles between pairs of planes.

Table 3-9 The dihedral angles (º) between pairs of planes for A. Plane 1 is defined by Cp ring, plane 2 is defined by P, C1, C2, plane 3 is defined by C1, O1, C2, C3, plane 4 is defined by Phenyl ring 1, plane 5 is defined by phenyl ring 2 and plane 6 is defined by phenyl ring 3

Plane Dihedral angle Plane Dihedral angle (˚) (˚) 1, 2 6.9(2) 4, 6 68.6(1) 1, 3 42.0(2) 4, 5 67.9(1) 1, 4 75.9(1) 5, 6 72.8(1) 1, 5 21.5(2) 2, 4 81.6(1) 1, 6 52.1(2) 2, 5 27.6(1) 2, 3 48.8(1) 2, 6 45.4(1)

The cell unit of A is presented below in Figure 3-3, to show the molecular packing in a unit cell.

Figure 3-3 Diamond [12] structure of the unit cell of A viewed along the b axis. The smaller box is the unit cell

71 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

The structure crystallizes in a triclinic crystal system and space group P1 . The unit cell of the structure of A presented above (Figure 3-3) shows the molecular packing of the unit cell. There are two molecules in the unit cell in agreement with the P1 space group. The two isomers of the unit cell are inverted mirror images of each other. Positional and thermal parameters of atoms for A are given in Table A.1-1 of Appendix.

5 3.3.2.5. The Crystal Structure of [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}], Acetyl(carbonyl)(η5-cyclopentadienyl)tri(p-fluorophenylphosphine)iron(II)

The molecular structure of B is presented below in Figure 3-4.

Figure 3-4 Diamond [12] plot of the molecular structure of B showing numbering of atoms; the first digit gives the ring number, the second digit gives the number of the atom in the ring. Thermal ellipsoids are drawn at 40% probability level.

72 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

The structure shows that the Cp ring is centrally bound above the plane of the metal. The bulky Cp ring pushes the carbonyl, acetyl and phosphine ligands away from the equatorial region of the complex such that they are pointing rather to the downward side like the legs of a piano stool in agreement with A (see Par. 3.3.2.4). The angle from the centre of the Cp through the Fe to P is 127.4(1) ˚, centroid-Fe-C(1) is equal to 124.7(1) ˚ and centroid-Fe-C(2) is 119.1(2) ˚. This shows that the phosphine ligand, which is more bulky compared to the carbonyl and acetyl, is pushed away most. The angles between these ligands are approximately 90 ˚, typical of octahedral coordination. Fe-centroid distance is 1.748(4) Å. The effective cone angle [13] is 152.4 ˚ (calculation illustrated in section A.3 of the Appendix) compared to the Tolman [14] cone angle of 145 ˚. The carbonyl ligand is almost linear with the Fe-C(1)-O(1) angle of 174.4(3).

73 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

Table of angles and bond distance of importance for complex B are presented below.

Table 3-10 Selected interatomic bond lengths (Å) and angles (º) for B with e.s.d. in parenthesis

Distances [Å]

Fe-C(1) 1.748(3) C(41)-C(42) 1.401(4) Fe-C(2) 1.956(3) C(41)-C(45) 1.426(4) Fe-C(43) 2.107(3) C(42)-C(43) 1.398(4) Fe-C(41) 2.115(3) C(43)-C(44) 1.412(5) Fe-C(45) 2.121(3) C(44)-C(45) 1.395(5) Fe-C(44) 2.122(3) P-C(11) 1.834(2) Fe-C(42) 2.125(3) P-C(21) 1.832(2) Fe-P 2.194(8) P-C(31) 1.841(2) C(1)-O(1) 1.151(3) C(14)-F(1) 1.361(3) C(2)-O(2) 1.203(3) C(24)-F(2) 1.352(3) C(2)-C(3) 1.518(4) C(34)-F(3) 1.359(3) Fe-centroid 1.748(4) C(43)-centroid 1.193(2) C(41)-centroid 1.201(1) C(44)-centroid 1.196(3) C(42)-centroid 1.193(2) C(45)-centroid 1.198(2)

Angles [˚]

C(1)-Fe-C(2) 94.41(12) C(11)-P-Fe 115.43(8) C(1)-Fe-P 91.72(9) C(21)-P-Fe 115.03(8) C(2)-Fe-P 90.16(8) C(31)-P-Fe 115.55(8) O(1)-C(1)-Fe 174.4(3) C(1)-Fe-centroid 124.72(1) O(2)-C(2)-C(3) 115.9(3) C(2)-Fe-centroid 119.09(1) O(2)-C(2)-Fe 124.8(2) P-Fe-centroid 127.36(1) C(3)-C(2)-Fe 119.2(2)

Torsion angle [˚]

C(1)-Fe-C(2)-O(2) 156.46(1) C(22)-C(21)-P-Fe 73.69(1) C(1)-Fe-C(2)-C(3) -27.24(1) C(32)-C(31)-P-Fe 52.46(1) C(12)-C(11)-P-Fe 18.66(1)

74 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

Table 3-11 below presents the dihedral angles between pairs of planes for complex B.

Table 3-11 Dihedral angles (º) between pairs of planes for B. Plane 1 is defined by Cp ring, Plane 2: defined by P, C1, C2, and Plane 3: defined by C1, O1, C2, C3, and Plane 4: defined by phenyl ring 1, Plane 5: defined by phenyl ring 2 and Plane 6: defined by phenyl ring 3

Plane Dihedral angle Plane Dihedral angle (˚) (˚) 1, 2 8.1(2) 4, 6 74.2(1) 1, 3 31.1(2) 4, 5 66.6(1) 1, 4 55.6(1) 5, 6 64.8(1) 1, 5 70.3(1) 2, 4 48.5(9) 1, 6 20.9(2) 2, 5 78.2(8) 2, 3 39.0(2) 2, 6 26.2(1)

75 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

The unit cell structure of B is shown in Figure 3-5 below.

Figure 3-5 Diamond [12] structure of the unit cell of B viewed along the b-axis. Thin dashed lines representing hydrogen bonding between the molecules connect the labelled atoms. The smaller box is the unit cell.

Compound B packs in the triclinic crystal system and space group P1 with two molecules in a unit cell. The formula weight of the complex agrees with this result. The molecules are vertically arranged head to tail with molecules from the next unit cells. Packing is stabilized by hydrogen bonding between fluorine atoms and the hydrogen atoms of the neighbouring molecules.

76 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

Table 3-12 below presents intermolecular hydrogen bonds distances in complex B.

Table 3-12 The Table of intermolecular hydrogen bond distances in complex B

Atoms Bond distances Atoms Bond distances (Å) (Å) F1-H42 2.520(0) F3-H35 2.520(5) F1-H15 2.536(6) F3-H33 2.534(3) F1-H23 2.904(4) F3-H16 2.673(4) F1-H43 3.083(4) F3-H33 2.752(3) F1-H13 2.508(5)

F2-H23 2.518(5) O1-H44 2.804(6) F2-H25 2.527(3) O2-H23 2.564(1) F2-H15 3.093(5) O2-H3C 2.372(4) F2-H33 2.538(6) O2-H3C 2.957(3)

3.3.3. Structural Correlations

Individually the two structures display similar geometric properties, therefore the sections below will be used to first compare the properties of these two structures and then correlate these with literature structures.

3.3.3.1. Correlation Between Structure of A versus B

In the previous sections, it was shown that the two compounds crystallized in a triclinic crystal system and space group P1 . In both cases the isomers are inverted mirror images of each other. Packing is stabilized by intermolecular hydrogen bonding in complex B.

Since the two structures’ geometry is so similar, the polyhedron data was employed to try to differentiate between the two. First of all interatomic bond distances of importance in this study were compared. The M-P bond of A is 2.193(8) Å and that of B is 2.194(8) Å. The two distances are basically the same, the difference falling within the calculated error. In both structures the Cp ring is centrally bound above the plane of the metal with an Fe-centroid 77 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION distance of 1.75(8) and 1.75(4) Å for A and B respectively. Other comparable bond distances are listed in Table 3-13. Most of these distances are almost the same, and one explanation could be that since the fluorine and methyl moieties, which are the distinguishing features of the two molecular structures, are on the periphery of the aryl phosphine ligands, and that the effect they induce on the M-P bond is insignificant. The angles between the centroid of the

Cp ring and the phosphine ligands going through Fe were also compared. P(p-MePh)3 in A is

126.3(2) ˚ and P(p-FPh)3 in B is 127.4(1) ˚ away from the Cp ring. There is a 1.1 ˚ increase in the angle centroid-Fe-P(p-FPh)3 contrary to the observation made with the effective cone angles. The effective cone angle of P(p-MePh)3 is 152.6 ˚ compared to 152.4 ˚ for P(p-FPh)3, since the two angles are equivalent it would be expected that the centroid–Fe-P angle remain the same for both ligands.

Torsion angle for C(3)-C(2)-Fe-C(1) of -13.28(2) and -27.24(1) ˚ was observed for A and B respectively; the difference being quite significant.

78 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

Table 3-13 compares the molecular dimensions of the two complexes of study A and B.

Table 3-13 Comparison of principal molecular dimensions of A and B. The e.s.d. values are recorded in parenthesis.

A B Distances [Å] Fe-C(1) 1.746(3) 1.748(3) Fe-C(2) 1.953(3) 1.956(3) Fe-P 2.193(8) 2.194(8) C(1)-O(1) 1.149(3) 1.151(3) C(2)-O(2) 1.202(3) 1.203(3) C(2)-C(3) 1.535(4) 1.518(4) C(14)-A 1.513(4) 1.361(3) C(24)-A 1.528(4) 1.352(3) Fe-centroid 1.751(7) 1.748(4)

Angles [˚]

C(1)-Fe-C(2) 94.70(12) 94.41(12) C(1)-Fe-P 93.04(8) 91.72(9) C(2)-Fe-P 89.00(8) 90.16(8) O(1)-C(1)-Fe 174.3(3) 174.4(3) Centroid-Fe-P 126.05(1) 127.36(1) Centroid-Fe-C(1) 124.21(2) 124.72(1) Centroid-Fe-C(2) 120.56(2) 119.09(1)

Torsion angle [º]

C(1)-Fe-C(2)-C(3) -13.28(2) -27.24(1) O(2)-C(2)-Fe-C(1) 170.23(2) 156.46(1)

Dihedral angles [º]

Plane 1 and 2a) 6.9(2) 8.1(2) a) Plane 1 is the plane of the Cp ring and plane 2 is described by P, C(1) and C(2).

In Table 3-13 above the dihedral angle between the plane of the Cp ring (plane 1) and the plane of the three ancillary ligands (plane 2) is 6.9(2) and 8.1(2) ˚ for A and B respectively. When taking into consideration the angle between the centroid, metal centre and the phosphine ligand, the tri(p-fluorophenyl)phosphine (127.4˚) is shifted away from the Cp ring more than the tri(p-tolyl)phosphine (126.28 ˚) is. It is expected that the bulkier phosphine ligand should have the largest dihedral angle between the two planes (plane 1 and plane 2) but the opposite is observed. This shows that the Cp ring-Fe moiety is very rigid because all these changes do not affect the arrangement of the carbon and hydrogen atoms making up the

79 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

Cp ring in relation to the metal centre. The average Fe-C(Cp) distances are similar in both structures, namely 2.113 and 2.118 Å for A and B respectively. In the following paragraphs these two structures are compared to other structures of the type found in literature to see if the same trend is observed.

3.3.3.2. Correlation with Literature Results

Extensive work has been done on the type of complexes under discussion. In order to rationalise certain parameters, such as steric bulk and electron donating capability, for ligands for which no or little data is available, it is important to cross-reference to other well-known ligand systems, ensuring that all ligands employed are compared on an identical scale [15], [26]. In this section the literature findings on tertiary aryl phosphine ligands are compared with the two complexes of this study.

Table 3-14 below presents the literature values of bond distances of interest compared with bond lengths of the two structure of this study. The phosphine ligands are not trans to any other coordinating ligand, therefore it can be concluded that the bond lengths are solely influenced by the electron density on the metal centre and not by a trans influence of some kind. Bulky phosphine compounds are not included in this table, thus steric effects on bond distances are eliminated. The Tolman cone angles are given in Table 3-15 below.

The Fe-P bond lengths of complex A and B are 2.193(8) and 2.194(8) Å respectively. These values are almost identical to those reported for all complexes in Table 3-13 below. Prock et al [22] predicted that Fe-P bond length is a good diagnostic indicator for σ-bonded or π- bonded ligands. These writers predicted that for σ-donor ligands Fe-P bond distance should be comparably long (around 2.20 Å) and for σ-donor/π-acceptor ligands should be comparably short (about 2.10 Å). Based on these arguments they classified PPh3 as a pure σ- donor. Comparison of the Fe-P bond distance in our structures with that of Fe-PPh3 would indicate that our phosphine ligands are pure σ-donors. The same argument was used for the

15 Roodt A. Otto S., Steyl G., Coord. Chem. Rev., 2003, 245, 121.

80 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION rest of the phosphine ligands found in Table 3-14 and they are all classified as pure σ-donors. The rest of the figures given in this table are also comparable, the difference falling within the calculated error. Complexes number 5, 8 and 10 have methyl substituents on the Cp ring, but that does not alter the iron to ring distances as compared to the two structures of this study.

5 Table 3-14 Selected geometrical parameters for complexes of the type [(η -C5H5)Fe(CO)(COMe){L}]

Complex Fe-P Fe- Fe- Fe- Fe-Cpa) Ref. C(≡O) C(=O) Centroid 5 1 [(η -C5H5)Fe(CO)- (COMe){Ph2PNH- CH(Me)(Ph)}] 2.198(66) 1.726(8) 1.950(65) 1.743(75) 2.110 [16] 5 2 [(η -C5H5)Fe(CO)- (COCH2C(OH)Ph){PPh3}] 2.199(15) 1.734(12) 1.952(24) 1.74(2) 2.106 [17] 5 3 [(η -C5H5)Fe(CO)- (COCH2Ph){PPh3}] 2.204(13) 1.706(12) 1.952(8) 1.737(4) 2.101 [18] 5 4 [(η -C5H5)Fe(CO)- (COCH(Me)Et){PPh3}] 2.193(20) 1.733(14) 1.965(1) 1.752(6) 2.121 [19] 5 5 [(η -C5H4Me)Fe(CO)- (COMe){PPh(Me)2}] 2.181(5) 1.726(23) 1.949(44) 1.739(28) 2.112 [20] 5 6 [(η -C5H5)Fe(CO)- This (COMe){P(p-FPh)3}] 2.194(8) 1.748(3) 1.956(3) 1.748(4) 2.118 study 5 7 [(η -C5H5)Fe(CO)- This (COMe){P(p-MePh)3}] 2.193(8) 1.746(3) 1.953(3) 1.751(7) 2.113 study 5 8 [(η -C5H4Me)Fe(CO)- (COMe{PPh2Et}] 2.200(2) 1.687(9) 1.925(10) 1.738(9) 2.110 [21] 5 9 [(η -C5H5)Fe(CO)- (COMe){PPh3}] 2.202(2) 1.708(2) 1.917(8) 1.752(5) 2.118(11) [21] 10 [(η5 -C5H4Me)Fe(CO)- 2.195(3) 1.694(10) 1.921(9) 1.746(8) 2.114(10) [21] (COMe){PPh3}] a) The average Fe-C(Cp) distances. b) All the values presented in this Table are in Å units.

16 Korp D. J., Bernal I., J. Organomet. Chem., 1981, 220, 355. 17 Liebenskind L. S., Welker M. E., Tetrahedron Lett., 1984, 25, 4341. 18 Krajewski J. W., Gluzinski P., Zamojski A., Mishnyov A., Kemme A., Guo Z., J.Crystallogr. Spectrosc. Res., 1992, 22, 213. 19 Baird G. J., Bandy J. A., Davies S. G., Prout K., J. Chem. Soc., Chem. Commun., 1983, 1202. 20 Liu H.Y., Koh L. L., Eriks K., Giering P W., Prock A., Acta Cryst., Sec. C: (Cr. Str. Com.) 1992, C48, 433. 21 Liu H.Y., Rahman M., Koh L. L., Eriks K., Giering P W., Prock A., Acta Cryst., 1989, C45, 1683.

81 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

Another factor influencing the coordination of the ligand is the steric size associated with the specific ligand. The previous chapter discussed the effect that steric properties have on the M-P bond lengths. Based on that theory, complexes with varying phosphorus(III) ligand Tolman cone angles are therefore compared to the effective cone angles of complexes A and B with their M-P bond lengths. The literature cone angle values for the two structures of the study (145 ˚) are much smaller than those found in this study, i.e., effective cone angle values of 152.4˚ and 152.6 ˚ determined for P(p-FPh)3 and P(p-MePh)3. This could be due to the use of Ni-P bond length to calculate the literature values. Since Ni is larger than Fe, it means a longer Ni-P bond than Fe-P bond; therefore Fe-P complexes would have a larger cone angle.

Table 3-15 Phosphine cone angles and Fe-P bond distances of the structures of the study and other 5 structures of complexes of the type [(η -C5H5)Fe(CO)(COMe)L] with the e. s. d. recorded in parenthesis.

Ligand Fe-P bond length Cone angle, θ, Ref. [Å] [˚]

PPh3 2.202 (2) 145 [22] PPhMe2 2.181 (14) 122 [20]

PPh2Me 2.185 (16) 136 [23] PPhMe2 2.175 (10) 122 [24] P(p-MePh)3 2.194(4) 152.6, (145) a)

P(p-FPh)3 2.193(8) 154.3, (145) a) PPh2Et 2.200(2) 136 [20], b) PPh3 2.215(6) 145 [25], c) PPh3 2.196(15) 145 [26], b) PPh3 2.211(14) 145 [27], b)

a) Complexes of study, b) Me substituent on the Cp ring and c) Has an indenyl ring instead of a Cp ring.

22 Liu Y. H., Koh L. L., Eriks K., Giering W. P., Prock A., Acta Cryst., Sec. C: (Cr. Str. Com.), 1990, C46, 51. 23 Rahman M., Liu Y. H., Prock A., Giering W. P., Eriks K., Koh L. L., Acta Cryst., Sec. C: (Cr. Str. Com.), 1992, C48, 433. 24 Herndon J., Chao W., Ammon H., J. Org. Chem., 1988, 53, 2873. 25 Davies S. G., Holland K. S., Sutton K. H., McNally J. P., Isr. J. Chem. 1991, 31, 25. 26 Liu Y. H., Rahman M. M., Eriks K., Giering W. P., Prock A., Organometallics, 1989, 8, 1. 27 Moromoto Y., Ando K., Uno M., Takahashi S., Chem. Commun., 1997, 1795.

82 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

The values of the M-P bond lengths given in Table 3-15 above, show weak dependence of M- P bond length on the steric properties of the coordinating phosphine ligands. Tolman showed that the M-P bond length is slightly dependent on the size of phosphorus(III) ligands. However it was later proved that the steric effect might be ushered in at a particular θ (steric threshold). As was observed by Trogler and Marzilli, the chemical shifts of methyl correlates linearly with cone angle. But the steric effects disappear for ligands with cone angles less than approximately 118 ˚, suggesting that there is a steric threshold near this value. Cotton and Darensburg noted that the structure of [(CO)5W{PMe3}] showed no signs of steric distortion, while [(CO)5W{P(t-Bu)3}] on the other hand is sterically distorted. This suggests that there must be a value of θ between cone angles for PMe3 and P(t-Bu)3 where steric effects become operative. Since the values compared above fall within the predicted range some other factors should be taken into consideration.

In the solid state it was not possible to distinguish between structure of complex A and B, therefore the liquid state should be explored to differentiate between the two structures. In order to evaluate the electronic characteristics of a specific ligand one needs a probe, related to electron density in some way, that can be measured conveniently and is sensitive to changes induced in the system by the specific ligand [15], [26].

A widely quoted parameter to indicate Lewis basicity of the tertiary phosphines is the pKa value thereof, which is the measure of the Brønsted basicity.

3.4. CORRELATION OF STRUCTURAL DATA WITH SPECTRO- SCOPIC FINDINGS

The nature of the metal phosphorus bond has a strong influence on the electron density of the metal centre. Strong σ-donors enhance the electron density whereas strong π-acids decrease the electron density.

83 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION

A summary of electronic parameters in iron complexes of the form [η-CpFe(CO)(COMe)L, Vaska type complexes and Tolman complexes is presented in Table 3-16 below.

Table 3-16 Table comparing electronic parameters for iron complexes of the form [η- CpFe(CO)(COMe)L], Vaska type complexes ([RhCl(CO)(L)2]) and the Tolman complexes [Ni(L)(CO)3].

νco Fe-CO νco νco θ b) Ref f) a) b) c) c) L pKa Fe (Å) Vaska Tolman (˚) e) P(OPh)3 -2.00 1951 1.740(3) - 2085 128 28 P(OMe)3 2.60 1939 - - 2079 107 P(OMe)2Ph 2.64 1934 - - - 120 P(OMe)Ph2 2.69 1926 - - 2072 132 P(p-ClPh)3 1.03 1926 - 1983 2073 145 P(p-FPh)3 1.97 1924 1.748(3) 1983 2071 154 This study PPh2Me 4.57 1924 1.700 - 2067 23 d) PPh2Et 4.90 1924 1.687(9) 1973 2067 140 29 PPh3 2.73 1922 1.708(2) 1979 2069 145 20 P(p-MePh)3 3.84 1920 1.746(3) 1976 2067 152 This study P(p-MeOPh)3 4.59 1919 - 1975 2066 145 d) PMe2Ph 6.50 1919 1.726(23) - 2065 122 30 PEt2Ph 6.25 1918 - 1964 2064 136 PCy2Ph - 1917 - 1964 2061 162 a) Ref. [31] and references therein b) Ref. [26], (νco values in cm-1) c) Ref. [14], [15] and references therein, (values in cm-1) d) Has -Me on the Cp ring e) The alkyl part of the acetyl is [-C(Me)=(Ph)Me] f) The references in the last column refer to the Fe-CO bond lengths only

Many literature [26], [32] studies show that kinetics and thermodynamics of certain organometallic reactions are linearly related to the Brønsted basicity (pKa), cone angle, θ, of ancillary or incipient phosphorus(III) ligands.

From the table above it can be seen that the pKa values increase with decreasing terminal carbonyl stretching frequencies. In theory the pKa values increases with increasing electron

28 Reger D. L., Mintz E., Lebioda., J. Am. Chem. Soc., 1986, 108, 8, 1940. 29 Liu Y. H., Prock A., Giering W. P., Eriks K., Acta Crys., Sec. C: (Cr. Str. Com.), 1989, C45, 1683. 30 Herndon J. W., Chao W., Ammon H. L., J. Org. Chem., 1988, 53, 2873. 31 Rahman M., Liu Y. H., Prock A., Giering W. P., Organometallics, 1987, 6, 650. 32 Golvin M., Rahman M., Belmonte J. E., Giering W. P., Organometallics, 1985, 4, 1981.

84 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION density while the inverse is true for the carbonyl stretching frequencies. As the electron donor ability of the phosphine ligands decreases, the electron density around the metal decreases accordingly, consequently there is less back donation of electrons from the metal centre into the carbon π*-orbitals. As a result the M-CO bond weakens and the ν(CO) increases. Based on that trend as depicted in Table 3-16 above it can be concluded that the electron donor capability increases from top to bottom in Table 3-16, making P(p-FPh)3 a good π-acceptor compared to P(p-MePh)3.

Since there is an increase in the electron density across the periodic table it would be expected that the Vaska and the Tolman complexes have lower ν(CO) compared to iron complexes due to enhanced π-back-bonding into the π* -orbitals. However, many factors contribute to this variation, making it more complex. First of all, the Tolman complexes have three carbonyl moieties decreasing the electron density around the metal centre, thus higher ν(CO) are observed. The Vaska type complexes on the other hand have -Cl groups which can withdraw electrons from the metal centre to some extent; thus the Vaska complexes also show higher ν(CO) values than the Fe complexes which in theory should have higher ν(CO). The charge on the metal centre would also favour high ν(CO) values for Fe complexes.

Several additional ways of evaluating the electronic properties of phosphine ligands are described in the literature [15]. A model (QALE) was proposed by Rahman et al. [26], which involves the use of linear relationship between the properties of the complexes and σ-donor/π- acidities of ligands. However, the details of the model are not deemed necessary for this study, therefore are not discussed further.

Chapter Four relates the effects of the crystallographic results on the rate at which the two ligands utilised in this study induce migratory insertion in the alkyl complex [2].

85 4 KINETICS AND MECHANISM OF THE MIGRATORY 5 CARBONYL INSERTION IN [(η -C5H5)Fe(CO)2Me]

4.1. INTRODUCTION

The first systematic analysis of reaction rates is attributed to Ludwig Wilhelmy who measured the rate of inversion of cane sugar in the presence of different acids by means of a polarimeter around 1850 [1]. Around 1910 it was realised that chemical reactions proceed through a series of consecutive elementary reaction steps. These individual steps combine to form the mechanism of the reaction and a fundamental requirement of an acceptable mechanism is that it must predict a rate law that is consistent with the experimental one. The rate law is an essential piece of mechanistic information because it contains the concentrations of species necessary to go from the reactant to the product by the lowest energy pathway. The study of the rates at which such chemical reactions take place is referred to as chemical kinetics. The tools for analysing the kinetics of such mechanisms include the steady state approximation, which was first applied by David Chapman in 1915 [1].

Chemical kinetics of migratory insertion has been studied extensively since the 1950s. The relevant theory has directly been discussed in Chapter Two. In the section that follows, the kinetic study of the migratory insertion in iron(II) complexes induced by aryl phosphines will be discussed based on NMR, IR and UV-visible studies. The results are used to propose a mechanism for the reaction and to obtain a rate law that best explains the mechanism. The results obtained are then compared to the results reported in literature.

1 van Santen R. A., Niemantsverdriet J. W., “Chemical kinetics and catalysis”, Plenum Press, New York, 1995, Chapter 1. CHAPTER 4 CHEMICAL KINETICS

4.2. RATE LAWS AND REACTION ORDERS

For the simple reaction depicted in Eq. (4-1) the rate of the reaction is defined by Eq. (4-2). k A + B → C (4-1)

− d[A] Rate = = k[A]a[B]b (4-2) dt

Here k denotes the rate constant and [A] and [B] are the concentrations of the reactants A and B. The sum of the superscripts a and b gives the order of the reaction. Under pseudo/approximate-first order conditions where [B] >> [A], the rate equation may be simplified as illustrated in Eq. (4-3).

− d[A] a Rate = = kobs[A] (4-3) dt

b In this case kobs is equivalent to k[B] . Rearrangement and integration (between [A]0, at time t

= 0 and [A]t at time = t) of Eq. (4-3) can help determine the order of the reaction, Eq. (4-1).

If a is equal to zero, the variables may be separated to obtain Eq. (4-4) which is then integrated to give Eq. (4-5).

d[A] = -kobsdt (4-4)

[A]t = [A]0 – kt (4-5)

For a first-order reaction in [A] rearrangement of Eq. (4-3) gives Eq. (4-6) below.

d[A]/[A] = -kobsdt (4-6)

The resulting equation may be integrated to give the following equation.

ln[A] = -kobst + C (4-7)

87 CHAPTER 4 CHEMICAL KINETICS

The constant of integration C is equal to the value of ln[A] at time, t = 0, which is ln of the initial concentration of A and may be represented by ln[A]0 so that Eq. (4-7) takes the form illustrated by Eq. (4-8) or Eq. (4-9)

ln[A]t = ln[A]0 -kobst (4-8)

log[A]t = log[A]0 – kt/2.3 (4-9)

At time t½ the rate constant, k is equal to 0.693 / t½ which is distinctive for first-order reactions.

If there is a second reaction taking place (e.g. reverse or a parallel reaction) then the rate of the reaction is defined by Eq. (4-10).

d[A]/dt = -k1[A][B] + k2[A] (4-10)

If the reaction is pseudo-second order with respect to A, the rate equation takes the form of Eq. (4-11) below.

d[A]/dt = -kobs[A] (4-11)

Here kobs = k1[B] + k2 (4-12)

Eq. (4-5), (4-8) and (4-12) are straight-line equations and they help determine the order of the reaction in A. For a zero order equation a plot of [A]t against time t also gives a straight line.

A first order reaction would give a straight line when ln[A]t is plotted against t; similarly the plot of kobs against [B] would classify a reaction as second order if the plot is a straight line that leads to a limiting value.

Experimentally it is not the concentration that is determined, but any factor that is proportional to concentration, such as the absorbance, the intensity of an NMR peak, etc.

88 CHAPTER 4 CHEMICAL KINETICS

The relation between the concentration, [A], of the absorbing species and the intensity of absorption, Abs, is given by Beer-Lambert’s law (Eq. (4-13)).

Abs = εcl (4-13)

Here Abs is optical absorbance, ε is molar absorptivity, c is concentration and l is light path length. If Abs is proportional to the concentration of substrate A (Abs α [A]) then [A]0 =

Abs0 - Abs∞ and [A]t = Abst - Abs∞, consequently substitution of this proportionality in the first order rate equation (Eq. (4-8)) gives Eq. (4-14). The rearrangement to the exponential form of this equation is illustrated by Eq. (4-15) below and is used to convert raw data into observed rate constants by non-linear, least squares analysis using a computer.

Abs0 − Abs∞  ln   = kobst (4-14)  Abst − Abs∞ 

(- k t ) Abst = Abs∞ - [Abs∞ - Abs0] exp obs (4-15)

4.3. ACTIVATION PARAMETERS

For a reaction to occur there should be a collision between ions involved. This collision results in activated species, which decompose into products. The energy that is required to form activated species is referred to as the energy of activation and is represented by the symbol Ea. The value of the rate constant is dependent on Ea, the frequency at which the collision occurs and the geometry of the system. Arrhenius illustrates this dependence with Eq. (4-16).

Ea k = ρZe- RT (4-16)

Here Z is the total number of collisions in 1 sec/mole/dm3, R is the universal gas constant, T is temperature and ρ is the steric factor.

89 CHAPTER 4 CHEMICAL KINETICS

From the logarithmic form of the Arrhenius equation, Eq. (4-17), ρZ and Ea are independent of temperature and therefore a plot of ln k against 1/T is usually linear with a slope equal to

Ea/R.

ln k = ln ρZ – Ea/RT (4-17)

For the reaction shown in Eq. (4-18) below the activated species, C‡, is believed to be in equilibrium with the reactants A and B, so that K‡ = [C‡] / [A][B] and the reaction is the product of equilibrium concentration of C‡ and the specific rate at which it decomposes.

‡ K k A + B C‡ D (4-18)

‡ The rate at which C decomposes is given by Eq. (4-19) below. Here kb is the Boltzmann’s constant and h is the Planck’s constant.

‡ ‡ Rate = kbT[C ]/h = kbTK [A][B]/h (4-19)

The experimental second order rate constant is given by the expression in Eq. (4-20) below.

‡ kexp = kbTK /h (4-20)

This rate constant can be expressed (Eq. (4-23)) using the thermodynamic constant given in Eq. (4-21).

∆G‡ = -RT lnK‡ = ∆H‡ - T∆S‡ (4-21)

Rearrangement of Eq. (4-21) gives Eq. (4-22).

‡ ‡ ‡ ‡  ∆G   ∆H   ∆S  K = exp −  = exp −  exp  +  (4.22)  RT   RT   R 

90 CHAPTER 4 CHEMICAL KINETICS

Substitution for K‡ into Eq.(4-20) gives the rate constant equation, Eq. (4-23)

‡ ‡ ‡ kbT  ∆G  kbT  ∆H   ∆S  kexp = exp −  = exp  −  exp  +  (4-23) h  RT  h  RT   R 

The logarithmic form of Eq. (4-23) gives the Eyring equation (Eq. (4-24)) upon rearrangement.

 k  k ∆S ‡ ∆H ‡  exp  b ln  = ln + − (4-24)  T  h R RT

‡ A plot of ln(kexp/T) against 1/T gives a straight line with the intercept (ln (kb/h) + ∆S /R) and slope -∆H‡/R.

4.4. EXPERIMENTAL PROCEDURE

4.4.1. General Data

Preparation of aliquots was done under nitrogen, but transfer of the solutions to cells was 5 5 done in open air. [(η -C5H5)Fe(CO)2]2, [1], and [(η -C5H5)Fe(CO)2Me], [2], were prepared as indicated in Chapter Three (section 3.2.2 and 3.2.3). The methyl complex [2] decomposes in both solid state and solution. In solid state it turns into brown impurities that did not dissolve in the organic solvents that was utilised (DCM and MeCN). No further purification was done prior to use. The P(p-FPh)3 and P(p-MePh)3 ligands were commercially (Aldrich and Merck) available and used as received. Solvents were dried according to standard literature procedures [2] as summarised in the Appendix (A.6.1 and A.6.2).

2 Perrin D. D., Armaredo L. F., “Purification of Laboratory Chemicals”, Butterworth-Heinemann Ltd, 1988, Oxford, Chapter 3. 91 CHAPTER 4 CHEMICAL KINETICS

1H, 19F and 31P NMR spectra were recorded on a Varian Gemini 2000 300 MHz NMR spectrometer. Chemical shifts (ppm) are relative to CDCl3 (7.24ppm), SO3CF3 (-75.206 ppm) 1 19 31 and H3PO4 (0.0 ppm) for H, F and P respectively. IR spectra were measured with a Bruker Equinox 55 FT-IR spectrometer and analysed with the OPUS system using 0.5 mm KBr solution cells. UV-visible and kinetic measurements were obtained on a Varian Cary 50 spectrophotometer equipped with water bath accurate to 0.1 ˚C in a double-sided 1 cm tandem quartz cell with teflon cap.

Kinetic runs and data analysis were conducted by means of Microsoft Excel 97 [3] and MicroMath Scientist [4] for Windows version 2 using Eq. (4-15).

4.4.2. Kinetic and Spectroscopic Studies

Kinetic measurements were carried out by UV-visible spectroscopy under pseudo/approximate-first order conditions, using a large excess of phosphine ligand. Blank experiments on solutions of the alkyl complex [2] in the absence of the ligand showed no significant decomposition during the time required for kinetic runs. Due to longer periods required to collect meaningful data, temperatures were maintained low to minimise evaporation of the solvent.

(a) Reaction of [2] with P(p-MePh)3

The batch method was also used to monitor the progress of the reaction between P(p-MePh)3 and complex [2] in MeCN. A mixture of complex [2] (31 mM) and P(p-MePh)3 (140 mM) with a total volume of six millilitres was heated at a temperature maintained at 58.0 ˚C and aliquots (0.10 ml) were taken at three-hour intervals during the day using a calibrated one cubic centimetre micro syringe. No sampling was done at night. Collected samples were stored in tightly closed vials and refrigerated below –10.0 ˚C before being analysed with IR, 1H and 31P NMR methods.

Complex [2] (0.52 mM) and a set of P(p-MePh)3 solutions ranging in [P(p-MePh)3] from 1.6 to 8.2 mM in MeCN were added in different sides of tandem quartz cells and the same kinetic analysis as that done for P(p-FPh)3, using UV-visible spectroscopy, was done in both MeCN and DCM.

(b) Reaction of [2] with P(p-FPh)3

A five mL reaction mixture with a final concentration of 31 mM of complex [2] and 120 mM 3 P(p-FPh)3 in MeCN was heated at a constant temperature (58.0 ˚C). Samples (0.10 cm ) were drawn from the reaction mixture using a calibrated one cubic centimetre micro syringe at approximately ten hour intervals during the day for the first three days (no sampling was done at night), then once a day or randomly until the reaction was considered complete. The collected aliquots were stored in tightly closed vials and refrigerated at a temperature below – 10.0 ˚C until they were analysed by IR, 19F NMR and 31P NMR methods.

One millilitre of complex [2] (0.52 mM) and one millilitre of P(p-FPh)3 ([P(p-FPh)3] between 5 and 150 mM) in MeCN were added into separate compartments of the tandem quartz cells. The cells were sealed with a teflon stopper greased with vacuum seal and placed into a thermostated cell holder. Time was allowed for thermal equilibration before the cells were shaken to ensure mixing of the two solutions. The resulting mixture was immediately placed in the spectrophotometer and the spectral change (as the reaction proceeds) was recorded in an overlay mode in the wavelength range of 200-600 nm. Five kinetic runs in duplicate were done simultaneously. From these wavelength profiles absorbance was obtained as a function of time and fitted in Eq. (4-15), generating rate constants.

Complex [2] (0.73 mM) and a set of PPh3 solutions ranging in [PPh3] from 5 to 150 mM in MeCN were added in different sides of tandem quartz cells and the same kinetic analysis as that done for P(p-FPh)3 and P(p-MePh)3, using UV-visible spectroscopy, was done in MeCN. Measurements were collected at two hours intervals (total time = 92 hrs.) at a wavelength range of 200 – 600 nm.

93 CHAPTER 4 CHEMICAL KINETICS

Since a dissociative mechanism is proposed pseudo first order conditions are not a prerequisite to observe first order kinetics. Iron concentrations needed to be as large as possible to enable NMR peaks to be integrated as accurate as possible. Consequently the range due to solubility of PX3 was limited.

Most of the products still contain the oxidised phosphine ligands after column chromatography and it was found that the oxidised ligand forms as the reaction proceeds. It was therefore of interest to determine what caused the oxidation.

(d) Analysis of the Formation of Oxidised Phosphine Ligands.

According to van Leeuwen [5] phosphine ligands can be oxidised by water, carbon dioxide and air. A nitrogen line was used to make sure that no air was introduced as the reaction proceeds. The solvents were dried prior to use. A blank reaction was done which contained only the ligand and the solvent under the same conditions as that of the reaction. The NMR spectrum of the ligand showed a negligible degree of oxidation.

In order to determine whether the oxidised phosphine was involved in the reaction or not, the oxidised P(p-FPh) ligand was reacted with the alkyl [2] using the same reaction conditions as with the non-oxidised ligand (58.0 ˚C, MeCN, under a nitrogen blanket). The ligand was analysed by NMR for the presence of any non-oxidised form of the ligand prior to the reaction and it was found that it is fully oxidised. After heating the reaction mixture for 20 hours the colour of the solution had changed from yellow to brown. 19F NMR and IR of the solution were taken to determine any sign of product being formed and to check if the alkyl has not decomposed in the process. The IR result showed that the alkyl species ([2]) did not undergo any decomposition, while the NMR results also showed that no reaction has occurred. More solvent and alkyl were added to increase the reaction mixture reactivity. Even after five days the NMR spectrum showed no indication of product. It was therefore concluded that the oxidised form of the ligand does not take part in the reaction whatsoever.

Another system was introduced in which the ligand was exposed to air to investigate the degree of oxidation in the presence of air. Before the reaction mixture was exposed to air it

5 van Leeuwen P. W. N. M., Applied Cat.: A Gen., 2001, 212, 61. 94 CHAPTER 4 CHEMICAL KINETICS was analysed by NMR to see if the solvent could have any effect on the ligand. The degree of oxidation of the ligand was considered to be negligible. Air was bubbled through the reaction mixture for three minutes and then left exposed to air. It was found that there was only ca. 7 % of the phosphine ligand oxidised after five days of heating, whereas in the presence of the alkyl complex [2], the same amount of oxidation is observed within seven hours. In five days, half of the amount of the phosphine ligand used is oxidised (see Figure 4-9 peak A and C).

One observation made during the reaction is that all excess phosphine ligand is oxidised as the reaction proceeds. Part of the product also decomposes into the oxidised form in solution. The decomposition product was not analysed further.

4.5. MECHANISTIC ASPECTS AND RATE LAW

4.5.1. Basic Arguments for Mechanistic Scheme and Rate Law

Different arguments were utilized as obtained from experimental measurements (this Chapter), characterization studies (Chapter 3) and literature (Chapter 2) in order to construct a complete mechanistic scheme.

5 5 The starting material (i.e. [((η -C5H5)Fe(CO)2)2] [1] and [(η -C5H5)Fe(CO)2Me)] [2]) 5 and the expected products [(η -C5H5)Fe(PX3)(CO)(COMe)] were well characterized by IR (see e.g. Figure 4-1), NMR (see e.g. Figure 4-2 for 31P, and 19F NMR see Figure 4-9), as well as X-ray crystallography in the case of products (section 3.3.2.4 and 3.3.2.5 where X is (p-MePh) and (p-FPh) respectively).

95 CHAPTER 4 CHEMICAL KINETICS

IR spectral change for the reaction between, for example, the alkyl complex, [(η5- 5 C5H5)Fe(CO)2Me], and the P(p-MePh)3 ligand, yielding an acetyl complex, [(η -

C5H5)Fe(CO)(COMe){P(p-MePh)3}], is illustrated in Figure 4-1. This shows the disappearance of the two bands A and B at 2006 and 1946 cm-1 (alkyl starting material) and the formation of a band C at 1915 cm-1 (acetyl product). There is an isosbestic point observed at ca.1930 cm-1.

A B 0.1

15 C 24

47 Absorbance Units 72 88

Wavenumber cm-1 5 Figure 4-1 The FT-IR spectral change for the reaction between [(η -C5H5)Fe(CO)2(Me)], 2, and P(p- 5 MePh)3 producing [(η -C5H5)Fe(COMe)(CO){P(p-MePh)3}] in MeCN at T = 58.0 ˚C. [P(p- 5 MePh)3] = 140 mM and [(η -C5H5)Fe(CO)2(Me)] = 31 mM. Numbers on the bands indicate the time intervals used for sampling in hours. Band A and B are the bands of complex [2] and band C is the band of the product. Kinetic data is given at a later stage in Table 4-1.

96 CHAPTER 4 CHEMICAL KINETICS

The 31P NMR spectral change for the reaction illustrated above in Figure 4-1 is

presented in Figure 4-2. The disappearance of the uncoordinated phosphine (δA = -

7.60 ppm), the formation of the acetyl species (δC = 73.69 ppm), as well as the

corresponding phosphine oxide (δB = 29.67 ppm previously discussed in Par. 4.4.2(d)), is observed. No intermediate species was detected by these NMR experiments.

-7.40 -7.60 -7.90 74.0 73.0 72.2 29.9 29.7 29.5

Time (hrs.)

δ (ppm)

31 5 Figure 4-2 P NMR spectral change for the reaction between [(η -C5H5)Fe(CO)2Me] (35 mM) and [P(p- 5 MePh)3] (100 mM) (A) yielding [(η -C5H5)Fe(COMe)(CO){P(p-MePh)3}] (C) in MeCN at T = 58.0˚C. Samples were drawn at time (hours) intervals indicated on the spectra (the first part of this Figure is the enlarged portion and the second part is the full spectra). B refers to OP(p- -5 -1 MePh)3. kobs ≈ 1.28 x 10 s with the estimated t½ value = 15 hrs.

97 CHAPTER 4 CHEMICAL KINETICS

The migratory insertion reaction could be monitored by UV-visible techniques since significant changes in the spectra of the different species were observed (see e.g. Figure 4-3 (MeCN) and Figure 4-4 (DCM)). The UV-visible spectral change for the

reaction between the alkyl complex [2] and P(p-MePh)3 in acetonitrile is presented in Figure 4-3 below.

0.90 0.30

0.25 a 0.75 0.20

0.15 0.60 0.10 0 50 100 150 200 10-3 Time (sec) 0.45 Absorbance 0.30

0.15

0.00 330 360 390 420 450 W avelength (nm )

5 Figure 4-3 The UV-visible spectral change for the reaction between [(η -C5H5)Fe(CO)2Me] (0.52 mM) and 5 [P(p-MePh)3] (26.3 mM) yielding [(η -C5H5)Fe(COMe)(CO){P(p-MePh)3}]. The reaction progress was monitored in MeCN at T = 48.6 ˚C over a wavelength range of 330 - 450 nm over the period of 43 hours with measurements recorded every hour (Last runs are not shown on the wavelength profile). Insert “a” is derived from absorbance measurements at λ = 450 nm. Data is presented in Table 4-2 and 4-3. The values in Table 4-2 for [P(p-MePh)3] = 16.5, 21.7 and 26.3 mM were determined at 360 nm because of the wider spectral change in that region but for the rest of the [P(p-MePh)3] were determined at 450 nm, which was the best wavelength for such low concentrations.

98 CHAPTER 4 CHEMICAL KINETICS

Variation of the solvents {DCM (ε = 9.1 [6], DN = 4 [7]) and MeCN (ε = 37.5, DN = 14.1 [6], [8])} with different polarity and donor ability yielded different results, and two distinct steps were observed in DCM (first more rapid reaction followed by second, with an isosbestic point at ca. 395 nm). See Figure 4-4 and Table 4-4 for kinetic data.

0.23 1.00 0.22

0.21 0.80 a b 0.20

0.19 0.60 0.18 0 50 100 150 200 250 10-3 Time (sec)

Absorbance 0.40

0.20

0.00 340 362 384 406 428 450 Wavelength (nm)

5 Figure 4-4 UV-visible wavelength profiles for the reaction between [(η -C5H5)Fe(CO)2Me] (0.64 mM) and 5 [P(p-MePh)3] (96 mM) yielding [(η -C5H5)Fe(COMe)(CO){P(p-MePh)3}]. The reaction was conducted in DCM at T = 30.1 ˚C over a wavelength range of 340 – 450 nm for the period of 68 hours at four hours intervals. The insert is a Scientist [3] plot of absorbance against time. It shows the two reactions “a” and “b”. λ = 400 nm (data is given at a later stage in Table 4-4).

6 Gutmann V., “The Donor-Acceptor Approach to Molecular Interactions”, Plenum Press, 1978, New York, Chapter 2. 7 Mzamane S. N., “MSc Thesis”, University of the Free State, 1999, Chapter 4. 8 Lide R. D., Frederikse H. P. R., ‘CRC Handbook of Chemistry and Physics”, 75th Ed., CRC Press Inc., London, 1994 - 1995, 8-66. 99 CHAPTER 4 CHEMICAL KINETICS

Based on the above arguments, as well as information from the literature [9], [10], [11], [12], [13], the scheme in Eq. (4-25) is proposed for the overall reaction.

K1, k1 k2 Alk + PX3 Int Acyl (4-25) k-1

5 5 Where Alk = [(η -C5H5)Fe(CO)2Me], Acyl = [(η -C5H5)Fe(COMe)(CO){PX3}] (here X = (p-

MePh) and (p-FPh)), Int = uncharacterised Intermediate species); K1 = equilibrium constant for rapid first reaction.

The rate law from Eq. (4-25), with [PX3] >> [Alk] reduces to Eq. (4-26) (for detailed derivation see section A.5).

k2 K1[PX 3 ] kobs = (4.26) 1 + K1[PX 3 ]

The formation of the intermediate species is given by Eq. (4-27).

kobs =k1 [PX3] + k-1 (4-27)

k1 Where K1 = (4.28) k-1

4.5.2. Results and Discussion

The experimental results for the variation of different parameters such as phosphine ligands and solvents are reported and discussed below.

9 Buttler I. S., Basolo F., Pearson R. G., Inorg. Chem., 1967, 6, 11, 2074. 10 Green M., Westlake D. J., J. Chem. Soc. (A), Inorg. Phys. Theor., 1971, 367. 11 Bassetti M., D., J. Am. Chem. Soc, 1993, 13, 4658. 12 Bassetti M., Mannina L., Monti D., Organometallics, 1994, 13, 3293. 13 Allevi M., Bassetti M., Lo Sterzo C., Monti D., J. Chem. Soc., Dalton Trans., 1996, 3527. 100 CHAPTER 4 CHEMICAL KINETICS

4.5.2.1. Insertion Induced by P(p-MePh)3

Examples of the spectral changes observed by IR and UV-visible for the P(p-MePh)3 induced migratory insertion were already shown in Figure 4-1 to Figure 4-4. A variation of phosphine concentration was done to determine the effect thereof on the reaction. The results from these reactions, including the kinetic evaluation of the individual traces (fitting by Scientist [3] to Eq. (4-15)) of the data, are described below.

The data obtained from IR measurements presented in Figure 4-1 are given in Table 4-1, and it is clear that the individual constants are in good agreement.

5 Table 4-1 The IR kinetic data for the disappearance of bands A and B of [(η -C5H5)Fe(CO)2Me] and the 5 formation of band C of [(η -C5H5)Fe(COMe)(CO){P(p-MePh)3}] (see Figure 4-1) for the reaction 5 between [P(p-MePh)3] = 140 mM and [(η -C5H5)Fe(CO)2Me] = 31 mM in MeCN at T = 58.0˚C. Bands A, B and C are observed at 2006, 1946 and 1915 cm-1 respectively. 6 Band wavenumber 10 kobs (cm-1) (s-1) A 2006 7.5 ± 1.0 B 1946 7.4 ± 0.9 C 1915 7.3 ± 1.2

101 CHAPTER 4 CHEMICAL KINETICS

A typical UV-visible wavelength profile for the reaction between P(p-MePh)3 and complex [2] in MeCN was presented in Figure 4-3 and the corresponding kinetic plots are presented in Figure 4-5.

Absorbance

-3 10 Time (sec)

5 Figure 4-5 UV-visible kinetic plots for the reaction between [(η -C5H5)Fe(CO)2Me] (0.52 mM) and a range of concentrations of P(p-MePh)3 (A = 1.6, B = 8.2, C = 11.0 and D = 21.0 mM) in MeCN at T = 50.0 ˚C. λ = 360 nm. 102 CHAPTER 4 CHEMICAL KINETICS

Kinetic data for the reaction of a range of concentrations of the P(p-MePh)3 and complex [2] (Figure 4-5) in MeCN are given in Table 4-2.

5 Table 4-2 The UV-visible data (kobs) for the reactions between [(η -C5H5)Fe(CO)2Me] (0.52 mM) and a 5 range of concentrations of P(p-MePh)3 yielding [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}] in MeCN. 5 5 [P(p-MePh)3] 10 kobs [P(p-MePh)3] 10 kobs (mM) (s-1) a) (M) (s-1) b) 1.6 2.31 ± 0.11 0.0082 2.2 ± 0.09a) 3.3 1.4 ± 0.07 0.0165 3.84 ± 0.13 4.9 2.94 ± 0.12 0.0217 2.86 ± 0.12 6.6 3.3 ± 0.09 0.0263 3.06 ± 0.15 a) Obtained at λ = 450 nm and T = 50.0 ˚C b) Obtained at λ = 360 nm and T = 48.6 ˚C

Concentration dependence of the observed rate constant derived from UV-visible kinetic data for the reaction between P(p-MePh)3 and complex [2], is presented in Figure 4-6 below.

0.05

0.04

0.03 3 -1 10 kobs (s ) 0.02

0.01

0.00 0.000 0.005 0.010 0.015 0.020 0.025 0.030

[P(p-CH3Ph)3] (M)

Figure 4-6 UV-visible kinetic plots showing the [P(p-MePh)3] dependence of pseudo/approximate-first 5 order rate constant for the reaction between [(η -C5H5)Fe(CO)2Me] (0.52 mM) and a range of 5 [P(p-MePh)3] yielding [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}] in MeCN. Points indicated by “●” were determined at λ = 360 nm and T = 48.6 ˚C and points indicated by “■” were determined at 450 nm and T = 50.0 ˚C.

103 CHAPTER 4 CHEMICAL KINETICS

The calculated value of the equilibrium constant (K1) obtained from the rate law (Eq. (4-26)) -1 -5 -1 is 300 M and the limiting value (k2) is (3.4 ± 0.4) x 10 s [data is quite inconsistent (see

Figure 4-6) therefore the value of K1 was fixed during the least square fit]. It is clear that the

K1 value is therefore not accurately determined from this set of data.

Within the large fluctuations in the data, there is still a reasonable agreement between the 5 observed rate constants for the formation of [(η -C5H5)Fe(COMe)(CO){P(p-MePh)3}] from 5 [(η -C5H5)Fe(CO)2Me] and P(p-MePh)3 when the reaction is followed by IR, NMR and UV- visible methods (See Table 4-3).

Table 4-3 The observed rate constants from different methods of analysis used to follow the reaction 5 5 progress between [(η -C5H5)Fe(CO)2Me] and P(p-MePh)3 yielding [(η -C5H5)Fe(CO)(CO- Me){P(p-MePh)3}]. IR (58.0 ˚C) 31P ( 58.0 ˚C) UV-vis (50.0 ˚C) [Ptol] = 140 mM [Ptol] = 120 mM [Ptol] = 15 mM 6 -1 a) b) c) 10 kobs (s ) 7.4 ± 1.0 ≈12.8 34 ± 0.4

a) Calculated from Figure 4-1 (average of the three values in Table 4-1) 31 b) determined from P NMR (Figure 4-2), with estimated t½ = 15 hrs c) UV-visible data from Figure 4-6, kobs = k2

When the same reaction between the alkyl complex [2] and P(p-MePh)3 is done in dichloromethane, the observed rate constants as reported in Table 4-4, are obtained.

5 Table 4-4 The UV-visible data (kobs) for the reaction between [(η -C5H5)Fe(CO)2Me] (0.64 mM) and a 5 range of concentration of P(p-MePh)3 yielding [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}] in DCM at T = 30.1 ˚C and λ = 360 nm. (See Figure 4-4 for a typical spectral change of the reaction) 6 [P(p-MePh)3] 10 kobs (mM) (s-1) 4.9 2.1 ± 0.4 40.0 1.3 ± 0.6 69.7 2.3 ± 1.4 96.0 1.5 ± 0.6

104 CHAPTER 4 CHEMICAL KINETICS

Fitting the data in Table 4-4 to the rate law equation (Eq. (4-26)) gives Figure 4-7 showing that the rate of the reaction increases with ligand concentration toward a limiting value. -1 -1 Values for k2 = 2.2 ± 0.5 s and K1 = 100 M were obtained. The value of K1 was fixed during least squares fit since the data showed large uncertainties.

3 6 -1 10 kobs (s ) 2

0 0.00 0.02 0.04 0.06 0.08 0.10

[P(p-MePh)3] (M)

Figure 4-7 UV-visible kinetic plot showing [P(p-MePh)3] dependence of pseudo/approximate-first 5 order rate constants for the reaction between a range of concentrations of P(p-MePh)3 and [(η - 5 C5H5)Fe(CO)2Me] (0.64 mM) producing [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}] in DCM at T = 30.1 ˚C and λ = 360 nm.

-6 -1 It seems as if the reaction in DCM with k2 = 2.2 ± 0.5 x 10 s , (therefore k2 is estimated to -6 -6 - be ≈ 8.8 x 10 at 50. 0˚C) is approximately 2-3 times slower than in MeCN (k2 = 36 x 10 s 1), thus an increase in polarity / donocity of the solvent does enhance the reaction rate.

Based on the kinetic results presented above it is concluded that the predicted reaction scheme, represented by Eq. (4-25), holds for migratory carbonyl insertion due to P(p-MePh)3. The derivation is given at a later stage [see Par. 4-7].

105 CHAPTER 4 CHEMICAL KINETICS

4.5.2.2. Insertion Induced by P(p-FPh)3

19 31 Insertion induced by P(p-FPh)3, was followed by IR, F NMR, P NMR and UV visible studies and the corresponding results will be presented in that order.

5 The IR spectral change for the reaction between the alkyl complex, [(η -C5H5)Fe(CO)2Me] 5 and P(p-FPh)3 producing an acetyl complex, [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}] is presented in Figure 4-8 below. This shows the disappearance of band A and B at 2006 and 1946 cm –1 (alkyl starting material) and band C and D at 1918 and 1652 cm-1 (acetyl product). There is an isosbestic point at ca. 1933 cm-1.

C 200 177 152 94 D Absorbance Units 53 20 5

Wavenumber cm-1

5 Figure 4-8 The FT-IR spectral change for the reaction between [(η -C5H5)Fe(CO)2Me] (31 mM) and P(p- 5 FPh)3 (120 mM) yielding [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}], in MeCN at T = 58.0 ˚C. Sample withdrawal was done at time intervals displayed on the Figure in hours. Bands A and B are the 5 bands for [(η -C5H5)Fe(CO)2Me], C and D are the bands for the product.

106 CHAPTER 4 CHEMICAL KINETICS

Kinetic plots of the IR data for the CO bands A and B of the starting alkyl complex [2] and 5 band C of the product, [(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}] are presented in Figure 4-9 below.

Absorbance

10-3 Time (sec)

5 Figure 4-9 FT-IR kinetic plots for the reaction between [(η -C5H5)Fe(CO)2Me] (31 mM) and [P(p-FPh)3] 5 (120 mM) in MeCN at T = 58.0 ˚C, yielding [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}]. A and B refer to the bands of the starting alkyl complex [2] and C is the band of the acyl product.

107 CHAPTER 4 CHEMICAL KINETICS

The data from the IR measurements for the reaction between Complex [2] and P(p-FPh)3 represented by Figure 4-8, is given in Table 4-5 below. There is a reasonable agreement between the data.

Table 4-5 FT-IR data for the reaction illustrated by (Figure 4-8) based on the disappearance of bands A 5 5 and B of [(η -C5H5)Fe(CO)2Me] and formation of band C of [(η -C5H5)Fe(COMe)(CO){P(p- FPh)3}] at T = 58.0 ˚C in MeCN. 6 Band Wavenumber 10 kobs (cm-1) (s-1) A 2006 6.7 ± 0.9 B 1946 6.2 ± 0.8 C 1918 1.7 ± 1.2

19F NMR spectral change for the reaction illustrated in Figure 4-8 is presented in Figure 4-10. This is characterised by the disappearance of the signal for the uncoordinated phosphine (A) at –114.25 ppm, the formation of the acetyl (B) at –112.30 ppm as well as the corresponding phosphine oxide (C) at –108.22 ppm. No intermediate species is observed.

Time (hrs.)

δ (ppm)

19 5 Figure 4-10 The F NMR spectral change during the reaction of [P(p-FPh)3], A, (120 mM) and [(η - 5 C5H5)Fe(CO)2Me] (31 mM) yielding [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}] (B) in MeCN at T = 58.0 ˚C. Samples were collected at the times (in hours) specified on the spectra. C is the corresponding phosphine oxide.

108 CHAPTER 4 CHEMICAL KINETICS

19 Scientist plots of the F NMR data for the reaction between complex [2] and P(p-FPh)3 are presented in Figure 4-11. A, B and C are the plots from the peaks of the uncoordinated phosphine, the acetyl and the corresponding phosphine oxide respectively.

NMR peak intensities (%)

10-6 Time (sec)

19 5 Figure 4-11 The F NMR kinetic plots for, free P(p-FPh)3 (A), the product [(η -C5H5)Fe(CO)(COMe){P(p- FPh)3}], B and P(p-FPh)3 oxide (C). These kinetic plots are derived from Figure 4-10, which 5 displays the reaction progress between [P(p-FPh)3] (120 mM) and [(η -C5H5)Fe(CO)2Me] (31 mM) giving B in MeCN at T = 58. 0 ˚C.

109 CHAPTER 4 CHEMICAL KINETICS

19F NMR Data derived from the Scientist plots [3] in Figure 4-11 are presented in Table 4-6 below.

19 5 Table 4-6 F NMR kinetic data (kobs) for P(p-FPh)3, A, [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}], B, and 5 OP(p-FPh)3, C, for the reaction between [(η -C5H5)Fe(CO)2Me] (31 mM) and P(p-FPh)3 (a range of concentrations) in MeCN at 58.0 ˚C (Figure 4-10 and 4-11). 6 10 kobs (s-1)

[P(p-FPh)3] P(p-FPh)3 [CpFe(CO)(COMe{L}] OP(p-FPh)3 (mM) (A) (B) (C) 120 4.2 ± 1.2 10.2 ± 1.2 2.0 ± 0.9 120 3.2 ± 0.4 6.8 ± 0.6 a) 1.7 ± 0.5 210 2.6 ± 0.3 14.5 ± 0.2 1.0 ± 0.8 270 1.6 ± 0.7 10.9 ± 3.0 2.8 ± 5.6 340 2.1 ± 0.5 12.0 ± 2.1 3.6 ± 3.6 a) Point plotted from a different kinetic run see Figure 4-12.

19 Again as was the case for the P(p-MePh)3, saturation kinetics is observed when the F NMR 5 data, for the formation of the acetyl complex [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}], was fitted to the rate law equation, Eq. (4-26) (see Figure 4-12).

0.020

0.015

3 -1 0.010 10 kobs (s )

0.005

0.000 0.00 0.07 0.14 0.21 0.28 0.35 [P(p-FPh)3] (M)

19 Figure 4-12 The F NMR kinetic plot showing the dependence of kobs values (Table 4-6) on [P(p-FPh)3] for 5 the formation of [(η -C5H5)]Fe(CO)(COMe){P(p-FPh)3}], B, from the solutions of different 5 concentrations of P(p-FPh)3 and [(η -C5H5)]Fe(CO)2Me] (30 mM) in MeCN at T = 58.0 ˚C. “•” is the point for the same concentration from a different kinetic run. 110 CHAPTER 4 CHEMICAL KINETICS

The pseudo/approximate-first order rate constants for the decrease of the free P(p-FPh)3 do not agree with a limiting model.

Figure 4-13 displays the 31P spectral change for the reaction represented by Figure 4-8 and 4-

10. The uncoordinated phosphine (A) is observed at δA = -8.5, the formation of the acetyl (B) at δB = 75.40 and the corresponding phosphine oxide (C) at δC = 27.5 ppm. It is clear that the spectra obtained are not as accurate as the 19F NMR data (Figure 4-10). The 31P were therefore only used as a qualitative method, confirming the formation of the acetyl complex as well as the corresponding phosphine oxide.

B C A

75.8 75.4 28.0 27.2 26.4 -8.0 -8.6 -9.0

Time (hrs.)

δ (ppm)

31 5 Figure 4-13. The P NMR spectral change for the reaction of [(η -C5H5)Fe(CO)2Me] (30 mM) and [P(p- 5 FPh)3], A, (120 mM) yielding [(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}], B, in MeCN at T = 58.0 ˚C. Numbers on the spectra indicates the time intervals used for sampling in hours. C is the corresponding phosphine oxide.

111 CHAPTER 4 CHEMICAL KINETICS

The plots of 31P NMR data fitted to Eq. (4-15) are represented below (Figure 4-14) for the 5 uncoordinated phosphine (A), the product B ([(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}]) and the corresponding phosphine oxide C.

NMR peak Intensity (%)

-3 10 Time (sec) 31 5 Figure 4-14 P NMR kinetic plots for the formation of the acetyl [(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}], 5 B, and the corresponding phosphine oxide (C) from a mixture of [(η -C5H5)Fe(CO)2Me] (30 mM) and [P(p-FPh)3], A, (12 mM) in MeCN at T = 58.0 ˚C.

112 CHAPTER 4 CHEMICAL KINETICS

31P NMR data obtained from kinetic plots in Figure 4-14 above are presented in Table 4-7 below. Data is in reasonably good agreement.

31 5 Table 4-7 The P NMR kinetic data for the reaction between P(p-FPh)3 (120 mM) and [(η - 5 C5H5)Fe(CO)2Me] (30 mM), yielding [(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}], in MeCN at T = 58.0 ˚C. 5 Peak 10 kobs (s-1)

A P(p-FPh)3 2.0 ± 0.3 B Acetyl 3.7 ± 0.8

C OP(p-FPh)3 2.1 ± 0.3

The UV-visible wavelength profile for reaction of complex [2] with P(p-FPh)3 in MeCN is presented in Figure 4-15. Insert “a” shows that there are two reactions (fast and slow).

0.8 1.20

1.05 0.7 b 0.90

0.6 0.75 a

0.60 0.5

Absorbance 0 100 200 300 0.45 10-3 Time (sec)

0.30

0.15

0.00 330 360 390 420 450 W a velen g th (n m )

5 Figure 4-15 UV-visible spectral change for the reaction between [(η -C5H5)Fe(CO)2Me] (0.52 mM) and 5 P(p-FPh)3 (150 mM) yielding [(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}] in MeCN at T = 50.0 ˚C. The reaction progress was monitored over the period of 74 hours with measurements recorded every two hours at a wavelength range of 330 - 450 nm. The insert shows the kinetic plot at λ = 360 nm. The last runs are not shown on the wavelength profile.

113 CHAPTER 4 CHEMICAL KINETICS

Scientist [4] plots for the fast reaction are presented below in Figure 4-16.

Absorbance

10-3 Time (sec) 5 Figure 4-16 UV-visible kinetic plots for the reaction between [(η -C5H5)Fe(CO)2Me] (0.52 mM) and a range of concentrations of P(p-FPh)3 (A = 5, B = 41, C = 79 and D = 150 mM) in MeCN at T = 50.0 ˚C. λ = 360 nm.

114 CHAPTER 4 CHEMICAL KINETICS

Table 4-8 presents the kobs values for the fast (Figure 4-16) and slow (see the Appendix, section A.8 Figure A.8-2 for the kinetic plots) reactions.

5 Table 4-8 The UV-visible data (kobs) for the reaction between [(η -C5H5)Fe(CO)2Me] (0.52 M) and a range 5 of concentrations of P(p-FPh)3 yielding [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}] in MeCN at T = 50.0 ˚C and λ = 360 nm. 4 6 [P(p-FPh)3] 10 kobs fast 10 kobs slow (M) (s-1) (s-1) 5 1.26 ± 0.08 0.2 ± 1.9 41 1.22 ± 0.07 1.4 ± 0.5 79 2.42 ± 0.02 1.7 ± 0.4 150 2.6 ± 0.6 1.8 ± 0.3

A limiting value is reached when UV-visible data is fitted to the rate law equation, Eq. (4-26). The respective plots are presented below in Figure 4-17.

0.5

0.4

3 -1 0.3 10 kobs (s ) 0.2

0.1 A 0.0 0.00 0.03 0.06 0.09 0.12 0.15 2.0

1.5

6 -1 10 kobs (s ) 1.0

0.5 B 0.0 0.00 0.030.06 0.09 0.12 0.15 [P(p-F Ph) ] (M) 3

Figure 4-17 Plot of UV-visible data showing dependence of the observed rate constant on [P(p-FPh)3] for 5 the reaction between [(η -C5H5)Fe(CO)2Me] (0.52 M) and a range of [P(p-FPh)3] in MeCN at T = 50.0 ˚C and λ = 360 nm. Data used is obtained from Table 4-8 and the parameters obtained from this Figure are presented in Table 4-9. A is determined from fitting the fast reaction data to Eq. (4-27) and B is determined from fitting the slow reaction data to Eq. (4-26). 115 CHAPTER 4 CHEMICAL KINETICS

Estimated equilibrium constant obtained from fitting the UV-visible data to a straight-line equation (Eq. (4-12) or (4-27)), is presented below in Table 4-9 together with the calculated value of the equilibrium constant obtained from the rate law equation (Eq. (4-26)). There is a reasonable agreement between the two values i.e., 8(4) versus 36(14) M-1 relatively. This data is obtained from the plots in Figure 4-17 presented above.

5 Table 4-9 UV-visible kinetic parameters for the fast and slow reaction between [(η -C5H5)Fe(CO)2Me] (0.00052 M) and a range of [P(p-FPh)3] in MeCN at T = 50.0 ˚C and λ = 360 nm. 4 4 6 10 k1 10 k-1 10 k2 K1 (M-1s-1) (s-1) (s-1) (M-1) Fast reaction 9.3 ± 4.0 1.2 ± 0.3 8 ± 4 a) Slow reaction - - 2.2 ± 0.2 b) 36.3 ± 14.0 b) a) Calculated from Eq. (4-28) b) Values determined from Eq. (4-26)

It is clear from Table 4-9 and Figure 4-17 that the model reasonably describes the kinetics, and also the rate law as given in Eq. (4-25) – (4-28).

The error that is associated with rate constants (k1 and k-1) for the fast reaction that were used to determine the equilibrium constant is incorporated into the equilibrium constant [14]. This effect can be very high especially in highly coordinating solvents and the use of less coordinating solvents helps to alleviate the problem. Therefore a less coordinating solvent, DCM, is used in this regard. However, the spectral change as observed for the reaction between the alkyl complex [2] and the P(p-FPh)3 in dichloromethane, DCM, was very small; (see Figure 4-18); therefore UV-visible techniques were not used to follow the reaction in DCM.

14 Roodt A., Otto S., Steyl G., Coord. Chem. Rev., 2003, 245, 121. 116 CHAPTER 4 CHEMICAL KINETICS

Figure 4-18 presents the UV-Visible spectral change for the reaction between complex [2] and P(p-FPh)3 in DCM.

1.20

1.05

0.90

0.75

0.60 Absorbance 0.45

0.30

0.15

0.00 330 360 390 420 450 W avelen gth (n m )

5 Figure 4-18 UV-visible spectral change for the reaction between [(η -C5H5)Fe(CO)2Me] (0.52 mM) and P(p-FPh)3 (78.1 mM) in DCM at T = 25.0 ˚C. The reaction progress was monitored over the period of 70 hours at two hours intervals at the wavelength range of 330-450 nm.

117 CHAPTER 4 CHEMICAL KINETICS

A summary of the respective rate constants for the reaction between P(p-FPh)3 and complex [2] is given below in Table 4-10. The observed rate constants obtained from 19F and 31P NMR and UV-visible analysis are in reasonable agreement within the calculated experimental error.

5 Table 4-10 Summary of rate constants for the formation of [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}] in MeCN using IR, NMR and UV-visible methods to monitor the reaction. IR (58.0 ˚C) 31P (58.0 ˚C) UV-vis 19F (58.0 ˚C)

[P(p-FPh)3] [P(p-FPh)3] (50.0 ˚C) [P(p-FPh)3]

= 120 mM = 120 mM [P(p-FPh)3] = 120 mM = 150 mM 5 -1 a) b) c) d) 10 kobs (s ) 0.5 ± 0.3 2.6 ± 0.6 26.0 ± 6.0 1.02 ± 0.12 5 -1 -1 e) 10 k1 (M s ) - - 93 ± 40 - 5 -1 e) 10 k-1 (s ) - - 12 ± 3 - 5 -1 e) f), g) 10 k2 (s ) - - 0.22 ± 0.02 1.2 ± 0.2

a) Calculated from Figure 4-8 and 4-9 (average of the three kobs values in Table 4-5) 31 b) Estimated from P NMR, Figure 4-13 and 4-14 (average of the three kobs values in Table 4-7) c) From UV-visible data, Table 4-8 for [P(p-FPh)3] = 150 mM d) Taken from Table 4-6 kobs for B when [P(p-FPh)3] = 120 mM e) Data from Table 4-9, Figure 4-17 f) Value of k2 from Eq. (4-26) with k2 = kobs g) Taken from Table 4-6 kobs for B when [P(p-FPh)3] = 340 mM

118 CHAPTER 4 CHEMICAL KINETICS

4.5.2.3. Insertion Induced by PPh3

Wavelength profile below (Figure 4-19) shows the UV-visible spectral change for the reaction 5 between [(η -C5H5)Fe(CO)2Me] and PPh3 in MeCN.

1 1.1

0.9 1.0 a 0.8 0.9

0.8 0.7

0.7 0.6 0.6 0.5 0 70 140 210 280 350 10-3 Time (sec) Absorbance 0.4

0.3

0.2

0.1

0 300 350 400 450 500 550 W avelength (nm )

5 Figure 4-19 UV-visible spectral change for the reaction between [(η -C5H5)Fe(CO)2Me] (0.73 mM) and PPh3 (150 mM) in MeCN at T = 48.8 ˚C in a wavelength range of 300-550 nm. Insert ‘a’ is a Scientist [3] plot at λ = 350 nm. The reaction progress was monitored over a period of 92 hours at 2 hours intervals.

It is evident from the insert in Figure 4-19 above that two reactions are also observed for the reaction between complex [2] and PPh3. Due to the lengthy time required for one to obtain a reasonable amount of data for kinetic analysis, the available data was only enough for qualitative analysis and thus the corresponding kinetic analysis will not be presented.

119 CHAPTER 4 CHEMICAL KINETICS

4.6. CORRELATION OF DATA FOR PX3 (X = (p-FPh) and (p-MePh))

Table 4-11 below summarises the rate constants obtained in this study that were used to elucidate the mechanism of the reaction. K1 is the equilibrium constant for the reaction, k2 is the rate constant for the coordination of the phosphine ligand, k1 is the rate constant for the formation of the intermediate species and k-1 is the rate constant for the corresponding back reaction (see Eq. (4-25)).

5 Table 4-11 Summary of reaction rate constants for the formation of [(η -C5H5)Fe(CO)(COMe){PX3}] from 5 [(η -C5H5)Fe(CO)2Me] and [PX3] (here X = (p-FPh) and (p-MePh)) in MeCN at T= 50.0 ˚C and DCM at T= 30.1 ˚C. a) b) 4 4 5 Solvent DN ε Ligand K1 10 k1 10 k-1 K1 10 k2 -1 -1 -1 -1 -1 -1 (M ) (M s ) (s ) (M ) (s ) c) c) MeCN 14.1 37.5 P(p-MePh)3 - - - 300 3.4 ± 0.4 d) e) e) f) P(p-FPh)3 8 ± 4 9.3 ± 4.0 1.2 ± 0.3 36.3 ± 14.0 0.22 ± 0.02

h) g) g) DCM 4 9.1 P(p-MePh)3 - - - 100 0.22 ± 0.05

a) Ref. [6], DN = donor number b) Ref. [8], ε = dielectric constant c) Eq. (4-26), Figure 4-6 d) Calculated from Eq. (4-28), Table 4-9 e) Values taken from Table 4-9 f) Calculated from Eq. (4-26), Figure 4-17 g) Calculated from Eq. (4-26), Figure 4-7 h) Estimated from chloroform

The rate of the reaction is phosphine dependent. From Table 4-11 above, P(p-MePh)3 is more reactive than P(p-FPh)3. This is in good agreement with the conclusion made in Chapter 3 that P(p-MePh)3 is a better σ-donor than the other two ligands it was compared to as well as the theoretical assumption that the migratory carbonyl insertion reaction is favoured by electron donating ligands.

The effective cone angle of P(p-MePh)3 (152.6 ˚) is equivalent to that of P(p-FPh)3 (152.4 ˚) but there is a significant difference in the reactivity of P(p-MePh)3 compared to P(p-FPh)3. It is therefore concluded that electronic effects are responsible for the reactivity of P(p-MePh)3.

120 CHAPTER 4 CHEMICAL KINETICS

Table 4-11 above also shows that the reactivity increases with the increasing polarity and donocity of the solvent since, for the P(p-MePh)3 ligand, reaction is faster in MeCN (which has a higher dielectric constant and higher donor ability (ε = 37.5 [8] and DN = 14.1 [6])) than

DCM which has a comparably lower ε (9.1) [6] and DN (4) [7].

Again, one can propose that, although the data in this study was not that conclusive, it is in agreement with the general mechanism proposed.

4.7. RATE LAW FOR THE PROPOSED REACTION SCHEME

4.7.1. Possible Schemes for the Reaction

Preliminary tests, results from the literature [9], [10], [11], [12], [13] as well as the more complete results presented above (Par. 4.5), enables the construction of Scheme 4-1 and 4-2 5 (Eq. (4-25)) illustrating the reaction between [(η -C5H5)Fe(CO)2Me] and P-donor ligands to 5 produce [(η -C5H5)Fe(COMe)(CO){PX3}]. Two possibilities can be visualised: i.e., an interchange process (Scheme 4-1) or a migratory insertion induced by solvent interaction, followed by coordination of a phosphine ligand (Scheme 4-2).

K1, k1 [(η5−CnHn)Fe(CO)2(Me)] + PX3 [(η5−CnHn)Fe(CO)2(Me), {PX3}]# k-1 k2

5 [(η −CnHn)Fe(COMe)(CO){PX3}]

5 Scheme 4-1 The proposed pathway that migratory insertion follows [11], [12], [13]. (η -CnHn) is either a Cp or indenyl ring.

121 CHAPTER 4 CHEMICAL KINETICS

K1, k1 [(η5−CnHn)Fe(CO)2(Me)] + S [(η5−CnHn)Fe(COMe)(CO)S]# + PX3 k-1 -S k2

5 [(η −CnHn)Fe(COMe)(CO){PX3}]

5 Scheme 4-2 The second proposed mechanism for migratory insertion [9[, [15]. (η -CnHn) is either a Cp or an indenyl ring.

Full derivation of the rate laws is given in the Appendix A.5, while arguments regarding these possibilities are presented below.

4.7.2. Formation of an Outer-sphere Intermediate Species (Scheme 4-1)

5 In these arguments, Fe is the alkyl complex, [(η -CnHn)Fe(CO)2(Me)], Fe(COMe) represents 5 the final product [(η -CnHn)Fe(COMe)(CO){PX3}], the tertiary phosphine PX3 is referred to 5 # as L and outer-sphere intermediate species, [(η -CnHn)Fe(CO)2(Me), {PX3}] is referred to as Fe,L#.

(i) If the rate-determining step is methyl migration, (Scheme 4-1) the rate of the reaction is given by Eq. (4-29).

d[Fe(COMe)] Rate = = k [Fe,L# ] (4-29) dt 2

Here [Fe(COMe)] is the concentration of the product, [Fe,L#] is the concentration of the outer sphere intermediate complex and k2 is the rate constant for the methyl migration step.

# [Fe]Tot = [Fe]Rem + [Fe,L ] (4-30)

15 Mawby R. J., Basolo F., Pearson R. G., J. Am. Chem. Soc., 1964, 86, 3994. 122 CHAPTER 4 CHEMICAL KINETICS

# [Fe]Tot is the total concentration of reacting iron complex in solution, [Fe,L ] is the concentration of the activated species and [Fe]Rem is the concentration of the reacting iron species that is not yet activated.

[Fe,L# ] K1 = (4-31) [Fe]Rem [L]

K1 is the equilibrium constant for the formation of the activated species and [L] is the concentration of the phosphine ligand in the reaction mixture.

Rearrangement of Eq. ((4-30) and (4-31)) and substitution into Eq. (4-29) with integration gives the rate of the reaction k (Eq. (4-32)).

k K [L][Fe] k = 2 1 Tot (4-32) 1 + K1[L]

Upon integration the observed rate constant is given by Eq. (4-33).

k2 K1[L] kobs = (4-33) 1 + K1[L]

Eq. (4-33 clearly indicates) two-step process and a consequent limiting rate value.

(ii) If the rate-determining step is the formation of the activated species, then the rate of the reaction is given by Eq. (4-34).

d[Fe(COMe)] Rate = = k [Fe,L# ] (4-34) dt 2

Applying steady state approximation followed by integration produces the rate of the reaction (Eq. (4-35)).

k k [L][Fe] k = 1 2 (4-35) k-1 + k2

123 CHAPTER 4 CHEMICAL KINETICS

Here k-1 is the rate constant for the reverse reaction of outer-sphere complex formation step. If [L] >>> [Fe] then [L] remains constant throughout the reaction, therefore the observed rate constant is given by Eq. (4-36).

k1k2 [L] kobs = (4-36) k-1 + k2

kobs is the pseudo-first order rate constant of the reaction.

Contrary to Eq. (4-33), Eq. (4-36) suggests a direct relationship between kobs and [L] and can thus be excluded based on the experimental results.

4.7.3. Formation of an Acyl Intermediate Species (Scheme 4-2)

5 # 5 Fe refers to the alkyl complex, [(η -CnHn)Fe(CO)2(Me)], Fe is the intermediate species, [(η - # 5 CnHn)Fe(COMe)(CO)] , Fe(COMe) is the final product, [(η -CnHn)Fe(COMe)(CO){PX3}] and the tertiary phosphine ligand, PX3, is referred to as L.

(i) Under conditions where the rate-determining step is methyl migration, (Scheme 4-2) the rate of the reaction is given by Eq. (4-37).

d[Fe(COMe)] Rate = = k [Fe# ][L] (4-37) dt 2

Here [Fe(COMe)] is the concentration of final product, k2 is the rate constant for phosphine coordination, [Fe#] is the concentration of intermediate species and [L] is the concentration of the reacting phosphine ligand.

Application of steady state approximation and integration gives Eq. (4-38).

k k [L][Fe] k = 1 2 (4.38) k-1 + k2 [L]

124 CHAPTER 4 CHEMICAL KINETICS k is the rate of the overall reaction, [Fe] is the concentration of the alkyl complex, [(η5-

CnHn)Fe(CO)2(Me)], k1 is the rate constant for the methyl migration step and k-1 is the rate constant for the decarbonylation reaction.

If [L] >>> [Fe] then the equation for the observed rate constant is shown below.

k1k2 [L] kobs = (4-39) k-1 + k2 [L]

At high ligand concentration, Eq. (4-39) simplifies to kobs = k1, which also suggest a limiting rate law, which is in agreement with the experimental results.

(ii) However, if the rate-determining step is a nucleophilic attack by the incoming phosphine ligand, then Eq. (4-40) gives the rate of the reaction.

d[Fe(COMe)] Rate = = k [Fe# ][L] (4-40) dt 2

# [Fe]Tot = [Fe]Rem + [Fe ] (4-41)

# Here [Fe]Tot is the total concentration of reacting iron species in solution, [Fe ] is the concentration of the activated species and [Fe]Rem is the concentration of the reacting iron species that is not yet activated.

[Fe# ] K1 = (4.42) [Fe]Rem

Rearrangement of Eq. (4-42) and substitution into Eq. (4-41) with integration gives the rate of the reaction as follows (Eq. (4-43)).

k K [L][Fe] k = 2 1 Tot (4-43) 1 + K1

[L] >>> [Fe]Tot

125 CHAPTER 4 CHEMICAL KINETICS

k2 K1[L] kobs = (4-44) 1 + K1

Eq. (4-44), as does Eq. (4-36), similarly suggests a direct relationship between kobs and [L] and can be excluded.

4.7.4. Proposed Final Mechanism and Rate Law

The following arguments were applied to determine the mechanism that best explains the reaction pathway.

Eq. (4-33) would best describe the reaction under the following conditions.

When high concentrations of the phosphine ligand are used Eq. (4-33) would simplify

to kobs = k2 and the value obtained would not be affected by the nature of the phosphine ligands.

Low phosphine concentration on the other hand would mean that KL[L] <<< 1 and therefore is neglected. As a result Eq. (4-33) would give a straight line that passes

through the origin (slope = k2K1).

At moderate concentrations of the ligand the value of K1[L] is comparable to 1,

therefore Eq. (4-33) retains its form and the graph of kobs against the concentration of the phosphine ligand is a straight line that reaches a maximum value at higher ligand concentrations.

If Eq. (4-36) were the ideal situation the same arguments would fit the experimental results obtained.

Except that at high concentrations of the phosphine ligand kobs = k1 and the value obtained would not change with varying phosphine ligands.

126 CHAPTER 4 CHEMICAL KINETICS

The slope that is obtained at low phosphine concentration would be equivalent to k k 1 2 . k-1

At moderate concentrations of the ligand the value of k2[L] is comparable to the value

of k-1, therefore Eq. (4-39) retains its form and the graph of kobs against the concentration of the phosphine ligand would be a straight line that levels off at higher ligand concentrations.

4.7.5. Conclusion

It is concluded, that a clear choice between Scheme 4-1 and 4-2 cannot be made, since the data is not that reliable. However, it is anticipated that Scheme 4-1, forming an outer-sphere complex, may be the more likely choice.

127 5 EVALUATION OF RESULTS AND FUTURE RESEACH

5.1. SCIENTIFIC EVALUATION OF THIS STUDY

Cyclopentadienyl carbonyl complexes of iron with different phosphine ligands were synthesised in fair yield for PPh3, P(p-FPh)3 and P(p-MePh)3, while the P(p-MeOPh)3 and

PPhCy2 were obtained in very low yield. IR, NMR, UV-Visible spectroscopy and X-ray crystallography were used to confirm the presence of the product.

The IR stretching frequencies ν(CO) of complexes with different phosphine ligands showed an increase in this order (PPhCy < P(p- MeOPh)3 < P(p- MePh)3 < PPh3 < P(p-FPh)3). In theory, CO stretching frequency increases with increasing π-acceptor property of the ligand

(see Table A.9-1 in the Appendix). Hence P(p-FPh)3 is a good π-acceptor compared to

PPhCy2 and that property increases from PPhCy2 to P(p-FPh)3. NMR results (see Table A.9-2 in the Appendix) however, do not follow a regular trend as the IR results.

5 5 The complexes [(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}] and [(η -C5H5)Fe(COMe)(CO)-{P(p-

MePh)3}] were successfully studied by X-Ray crystallography. The resulting complexes crystallize in the triclinic crystal system and P1 space groups. When other properties such as Fe-P bond lengths, which are believed to be sensitive to the electron density around the metal centre, are used to differentiate between the phosphine ligands of this study, little or no valuable information is obtained.

Based on the kinetic experimental analysis as well as the literature [1], [2], [3], [4], [5], the following reaction scheme (Eq. (5-1)) was suggested for the overall reaction.

1 Buttler I. S., Basolo F., Pearson R. G., Inorg. Chem., 1967, 6, 2074. 2 Green M., Westlake D. J., J. Chem. Soc. (A), Inorg. Phys. Theor., 1971, 367. 3 Bassetti M., D., J. Am. Chem. Soc, 1993, 13, 4658. 4 Bassetti M., Mannina L., Monti D., Organometallics, 1994, 13, 3293. 5 Allevi M., Bassetti M., Lo Sterzo C., Monti D., J. Chem. Soc., Dalton Trans., 1996, 3527. CHAPTER 5 SCIENTIFIC EVALUATION

K1, k1 k2 Alk + PX3 Int Acyl (5-1) k-1

5 5 Where, Alk = [(η -C5H5)Fe(CO)2Me], Acyl = [(η -C5H5)Fe(COMe)(CO){PX3}] (here X = (p- MePh) and (p-FPh), Int = uncharacterised intermediate species).

5 The kinetic analysis of the migratory insertion in [(η -C5H5)Fe(CO)2Me] induced by P(p-

FPh)3 and P(p-MePh)3 showed that the reactivity is dependent on the solvent. Even though there were great fluctuations in the experimental results, one magnitude [k2 (DCM ε = 9.1, DN -6 -1 -5 -1 = 4) = (2.2 ± 0.5) x 10 s and k2 (MeCN ε = 37.5, DN = 14.1) = (3.4 ± 0.4) x 10 s ] increase in the reactivity of P(p-MePh)3 was observed when the medium in which the reaction is conducted is switched from DCM to MeCN.

From the limited amount of kinetic data obtained in this study it was possible to show that -5 P(p-MePh)3 reacts faster than P(p-FPh)3 [k2 (P(p-MePh)3) = (3.4 ± 0.4) x 10 and k2 (P(p- -6 FPh)3) = (2.2 ± 0.2) x 10 ].

A recurring problem in this study is that due to extended periods (> 10 days) required for the reaction to proceed, evaporation of the reaction medium presented completion of the reaction under inconsistent conditions. Having to refrigerate the samples before analysis led to reactions taking place in the batch samples, interfering with the results.

According to literature [1], [2], [6] and [7] the reactions under investigation are solvent dependent being faster in stronger donor solvents such as MeCN (DN = 14.1), therefore

MeCN was mostly used in kinetic analysis, but it showed low solubility for the P(p-MePh)3 ligand.

A second solvent that was used to this effect was DCM. It has a lower dielectric constant and donor number (ε = 9.1 and DN = 4.0) compared to MeCN with a higher dielectric constant and

6 McFarlane K. L., Ford P. C., Organometallics, 1998, 1166. 7 Bibler J. P., Wojcicki A., Inorg. Chem., 1966, 5, 889.

129 CHAPTER 5 SCIENTIFIC EVALUATION a donor number (ε = 37.5 and DN = 14.1).

This solvent however gave small spectral change for the P(p-FPh)3 ligand, making it difficult to study the kinetic progress, even though the solubility was very good for both P(p-FPh)3 and

P(p-MePh)3.

5.2. FUTURE RESEARCH

A number of recent papers [1], [2], [5], [6], [7], [8] have discussed the influence of the alkyl group, the solvent, the nature of the metal, and the nucleophile, PX3, on the rate of carbonyl insertion. The effect of the nucleophile on the rate was mainly investigated on the tertiary phosphine. The research on the influence of the other group 15 tertiary ligands (arsine and stibine tertiary ligands) on the rate of CO insertion is still ongoing. Future research will focus on such ligands exploiting a wider range of solvent with higher boiling points to enable studying the reactions at higher temperature thus increasing the reactivity. This will also allow for a good choice of solubility.

Studying more crystals with the group 15 tertiary ligands will give an extensive insight into the steric effect which were observed for the P(p-MePh)3 to see if higher rates observed with this ligand compared to P(p-FPh)3 are solely due to electronic properties. This knowledge will help to fine-tune the ligands in a way that will increases the reactivity and thus help design ligands that are of importance in catalysis.

Literature [9] illustrated that using an indenyl ring instead of a cyclopentadienyl ring enhances the reactivity of this type of complexes towards migratory carbonyl insertion; therefore a wider range of substituents on the Cp ring can be explored, especially those that increase the electron density around the metal centre. Coupling this to a variety of alkyl chain lengths, which are also believed to have a positive influence on the reactivity of the

8 Anderson J., Moss J. R., George R., J. Organomet. Chem., 1995, 505, 131. 9 Mawby R. J., Hart-Davis A. J., J. Am. Chem. Soc., 1969, 2403.

130 CHAPTER 5 SCIENTIFIC EVALUATION complexes, will also, be worthwhile.

Other researchers focused their attention on thermally induced migratory insertion in the absence of organic solvents; it is therefore also of interest to determine the effect of pressure on such reactions in the absence of a solvent using a high-pressure reactor coupled to a FT-IR spectrometer.

Since these complexes can easily be reduced and oxidised without decomposing in the process it might also be worthwhile to study migratory insertion quantitatively by electrochemistry.

131 APPENDIX

5 A.1. CRYSTAL DATA OF [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}]

Table A.1-1 Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2 x 103) for 5 [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}]. Ueq is defined as one third of the trace of orthogonalized Uij tensor. X Y Z U(eq) Fe 2436(1) 4288(1) 3298(1) 49(1) C(1) 4419(3) 5206(3) 3654(2) 60(1) O(1) 5691(3) 5801(3) 3950(1) 91(1) C(2) 2435(3) 4943(3) 2320(2) 60(1) O(2) 1401(3) 4482(2) 1768(1) 79(1) C(3) 3769(4) 6191(4) 2239(3) 109(1) C(41) 1615(4) 4492(5) 4468(2) 91(1) C(42) 1194(4) 5641(4) 4184(2) 86(1) C(43) 124(3) 5002(3) 3498(2) 73(1) C(44) -75(3) 3489(3) 3345(2) 67(1) C(45) 827(4) 3170(4) 3948(2) 76(1) P 3310(1) 2281(1) 2608(1) 40(1) C(11) 4643(3) 2449(2) 1791(1) 43(1) C(13) 7304(3) 3430(3) 1434(2) 60(1) C(12) 6197(3) 3283(3) 2010(1) 53(1) C(14) 6890(4) 2770(3) 622(2) 64(1) C(15) 5345(4) 1958(3) 408(2) 73(1) C(16) 4222(3) 1781(3) 972(1) 61(1) C(17) 8095(5) 2915(4) -11(2) 98(1) C(21) 1663(3) 735(2) 2134(1) 46(1) C(22) 481(3) 917(3) 1595(2) 55(1) C(23) -793(3) -215(3) 1245(2) 60(1) C(24) -956(3) -1560(3) 1415(2) 59(1) C(25) 192(3) -1733(3) 1955(2) 60(1) C(26) 1487(3) -609(3) 2315(2) 53(1) C(27) -2351(4) -2818(3) 1014(2) 90(1) C(31) 4641(3) 1470(2) 3174(1) 46(1) C(32) 5581(3) 447(3) 2771(2) 59(1) C(33) 6616(3) -141(3) 3200(2) 68(1) C(34) 6738(3) 287(3) 4036(2) 62(1) C(35) 5794(3) 1296(3) 4435(2) 65(1) C(36) 4756(3) 1902(3) 4016(1) 55(1) C(37) 7911(4) -341(4) 4492(2) 94(1)

APPENDIX

Table A.1-2 Hydrogen coordinates (x 104) and isotropic displacement parameters (A2 x 103) for [(η5- C5H5)Fe(CO)(COMe){P(p-MePh)3}]. x y z U(eq) H(3A) 3570 6401 1736 175(5) H(3B) 4826 5893 2253 175(5) H(3C) 3745 7061 2679 175(5) H(41) -705 2799 2914 76(2) H(42) 2300 4599 4924 76(2) H(43) 1561 6645 4412 76(2) H(44) -369 5510 3194 76(2) H(45) 890 2232 3994 76(2) H(13) 8344 3984 1598 76(2) H(12) 6502 3751 2553 76(2) H(15) 5040 1508 -137 76(2) H(16) 3189 1217 804 76(2) H(17A) 7869 3677 -242 175(5) H(17B) 7986 1995 -427 175(5) H(17C) 9193 3166 236 175(5) H(22) 552 1811 1471 76(2) H(23) -1563 -70 885 76(2) H(25) 102 -2625 2082 76(2) H(26) 2242 -757 2681 76(2) H(27A) -2323 -3624 1242 175(5) H(27B) -2223 -3145 443 175(5) H(27C) -3384 -2475 1103 175(5) H(32) 5520 151 2209 76(2) H(33) 7233 -831 2920 76(2) H(35) 5848 1580 4998 76(2) H(36) 4143 2595 4298 76(2) H(37A) 7684 -1400 4309 175(5) H(37B) 7765 -15 5061 175(5) H(37C) 9022 -4 4394 175(5)

133 APPENDIX

2 3 5 Table A.1-3 Anisotropic displacement parameters (A x 10 ) for [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}]. The anisotropic displacement factor exponent takes the form: -2 pl2 [h2 a*2 U11 + …2 h k a* b* U12] U11 U22 U33 U23 U13 U12

Fe 39(1) 51(1) 55(1) 8(1) 10(1) 12(1) C(1) 45(1) 60(1) 64(2) -3(1) 7(1) 8(1) O(1) 59(1) 103(2) 89(2) -5(1) -8(1) 2(1) C(2) 48(1) 57(1) 82(2) 26(1) 11(1) 22(1) O(2) 73(1) 81(1) 86(1) 34(1) -7(1) 5(1) C(3) 75(2) 102(3) 175(4) 89(3) 5(2) 4(2) C(41) 43(1) 76(2) 78(2) 11(1) 14(1) 10(1) C(42) 58(2) 161(4) 51(2) 16(2) 16(1) 32(2) C(43) 58(2) 86(2) 91(2) -18(2) 24(2) 15(2) C(44) 50(2) 84(2) 89(2) 19(2) 17(1) 30(1) C(45) 61(2) 95(2) 85(2) 39(2) 34(2) 23(2) P 36(1) 46(1) 41(1) 12(1) 3(1) 11(1) C(11) 42(1) 48(1) 41(1) 13(1) 6(1) 14(1) C(13) 46(1) 69(2) 67(2) 23(1) 16(1) 12(1) C(12) 45(1) 65(1) 48(1) 12(1) 7(1) 12(1) C(14) 69(2) 72(2) 62(2) 27(1) 24(1) 26(1) C(15) 89(2) 87(2) 42(1) 12(1) 12(1) 19(2) C(16) 60(2) 73(2) 45(1) 13(1) 4(1) 7(1) C(17) 106(3) 114(3) 78(2) 31(2) 49(2) 23(2) C(21) 41(1) 46(1) 49(1) 11(1) 6(1) 10(1) C(22) 51(1) 51(1) 65(2) 22(1) -6(1) 7(1) C(23) 49(1) 62(2) 69(2) 20(1) -9(1) 6(1) C(24) 47(1) 54(1) 71(2) 12(1) 8(1) 2(1) C(25) 58(2) 52(1) 75(2) 27(1) 10(1) 6(1) C(26) 48(1) 55(1) 61(1) 23(1) 6(1) 13(1) C(27) 70(2) 70(2) 120(3) 24(2) -12(2) -14(2) C(31) 39(1) 53(1) 50(1) 19(1) 1(1) 10(1) C(32) 57(1) 68(2) 56(1) 20(1) 5(1) 24(1) C(33) 55(2) 77(2) 84(2) 35(2) 9(1) 27(1) C(34) 45(1) 73(2) 79(2) 41(2) -9(1) 4(1) C(35) 68(2) 77(2) 55(2) 29(1) -10(1) 7(1) C(36) 54(1) 63(1) 50(1) 19(1) 0(1) 13(1) C(37) 65(2) 116(3) 124(3) 72(2) -19(2) 15(2)

134 APPENDIX

5 A.2. CRYSTAL DATA OF [(η -C5H5)Fe(CO)(COMe)P(p-FPh)3]

Table A.2-1 Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2 x 103) for 5 [(η -C5H5)Fe(CO)(COMe)P(p-FPh)3]. Ueq is defined as one third of the trace of orthogonalized Uij tensor. X Y Z U(eq) Fe 3083(1) 2028(1) 3463(1) 39(1) C(1) 4664(3) 2383(3) 3963(2) 50(1) O(1) 5681(3) 2497(3) 4332(1) 79(1) C(2) 3046(3) 4145(3) 2840(2) 50(1) O(2) 2545(3) 4501(3) 2201(1) 69(1) C(3) 3588(6) 5479(4) 3195(3) 96(1) C(41) 842(3) 1173(3) 3299(2) 55(1) C(42) 478(3) 2349(3) 3788(2) 53(1) C(43) 1145(3) 1906(4) 4505(2) 60(1) C(44) 1887(4) 411(4) 4479(2) 69(1) C(45) 1712(3) -46(3) 3739(2) 61(1) P 4869(1) 1294(1) 2430(1) 35(1) C(11) 6329(3) -267(3) 2725(1) 38(1) C(12) 5976(3) -1205(3) 3504(2) 44(1) C(13) 7039(4) -2418(3) 3735(2) 52(1) C(14) 8456(4) -2653(3) 3189(2) 55(1) C(15) 8871(4) -1765(3) 2418(2) 56(1) C(16) 7792(3) -579(3) 2186(2) 48(1) F(1) 9523(3) -3807(2) 3434(1) 89(1) C(21) 6295(3) 2800(3) 1833(2) 39(1) C(22) 7569(3) 3206(3) 2194(2) 47(1) C(23) 8656(3) 4367(3) 1795(2) 55(1) C(24) 8434(4) 5145(3) 1035(2) 60(1) C(25) 7194(4) 4809(4) 664(2) 69(1) C(26) 6125(3) 3621(3) 1065(2) 55(1) F(2) 9494(3) 6287(2) 645(1) 90(1) C(31) 3969(3) 555(3) 1648(2) 41(1) C(32) 2762(3) 1472(3) 1285(2) 49(1) C(33) 2017(4) 941(4) 717(2) 56(1) C(34) 2452(4) -520(4) 524(2) 55(1) C(35) 3575(4) -1474(4) 880(2) 62(1) C(36) 4360(4) -921(3) 1441(2) 51(1) F(3) 1714(3) -1045(2) -34(1) 81(1)

135 APPENDIX

Table A.2-2 Hydrogen coordinates (x 104) and isotropic displacement parameters (A2 x 103) for [(η5- C5H5)Fe(CO)(COMe)P(p-FPh)3]. X y z U(eq) H(3A) 4761 5398 3173 144 H(3B) 3028 5432 3761 144 H(3C) 3320 6452 2875 144 H(41) 567 1184 2782 65 H(42) -109 3270 3658 63 H(43) 1106 2492 4925 72 H(44) 2403 -169 4885 82 H(45) 2095 -980 3564 74 H(12) 5013 -1014 3874 53 H(13) 6789 -3055 4250 63 H(15) 9852 -1957 2061 67 H(16) 8042 24 1661 57 H(22) 7687 2680 2716 56 H(23) 9513 4614 2035 66 H(25) 7067 5366 151 83 H(26) 5284 3374 814 65 H(32) 2463 2446 1431 59 H(33) 1237 1557 468 68 H(35) 3813 -2469 752 74 H(36) 5151 -1545 1678 61

136 APPENDIX

2 3 5 Table A.2-3 Anisotropic displacement parameters (A x 10 ) for [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}]. The anisotropic displacement factor exponent takes the form: -2 pl2 [h2 a*2 U11 + …2 h k a* b* U12] U11 U22 U33 U23 U13 U12 Fe 31(1) 39(1) 46(1) -4(1) -5(1) -3(1) C(1) 43(1) 61(2) 45(1) -9(1) -2(1) -4(1) O(1) 57(1) 124(2) 66(1) -21(1) -28(1) -4(1) C(2) 34(1) 42(1) 68(2) -1(1) -6(1) 3(1) O(2) 75(2) 56(1) 72(1) 5(1) -23(1) 8(1) C(3) 112(3) 47(2) 143(4) -13(2) -60(3) -6(2) C(41) 37(1) 59(2) 68(2) -12(1) -9(1) -9(1) C(42) 34(1) 52(2) 70(2) -8(1) -5(1) 0(1) C(43) 39(1) 85(2) 52(2) -12(2) 3(1) -10(1) C(44) 41(2) 83(2) 65(2) 24(2) -1(1) -5(2) C(45) 40(2) 41(2) 97(2) -6(2) 1(2) -8(1) P 32(1) 37(1) 37(1) -1(1) -11(1) -2(1) C(11) 37(1) 38(1) 41(1) -4(1) -12(1) -1(1) C(12) 40(1) 47(1) 44(1) -4(1) -11(1) 4(1) C(13) 62(2) 45(1) 49(2) -1(1) -18(1) 11(1) C(14) 58(2) 44(1) 68(2) -14(1) -25(1) 19(1) C(15) 45(2) 53(2) 67(2) -14(1) -3(1) 10(1) C(16) 46(1) 48(1) 47(1) -5(1) -6(1) 4(1) F(1) 85(1) 72(1) 103(2) -7(1) -25(1) 45(1) C(21) 32(1) 41(1) 41(1) -1(1) -6(1) -2(1) C(22) 41(1) 51(2) 47(1) 2(1) -12(1) -10(1) C(23) 43(2) 56(2) 64(2) -1(1) -13(1) -13(1) C(24) 51(2) 50(2) 71(2) 10(1) -4(1) -15(1) C(25) 72(2) 70(2) 57(2) 23(2) -18(2) -18(2) C(26) 50(2) 62(2) 50(2) 9(1) -18(1) -13(1) F(2) 79(1) 80(1) 98(2) 30(1) -13(1) -42(1) C(31) 40(1) 43(1) 40(1) -1(1) -11(1) -7(1) C(32) 48(2) 48(1) 55(2) -5(1) -21(1) -4(1) C(33) 52(2) 64(2) 55(2) 4(1) -24(1) -12(1) C(34) 57(2) 68(2) 45(2) -7(1) -18(1) -23(1) C(35) 68(2) 59(2) 65(2) -22(2) -19(2) -8(2) C(36) 54(2) 48(2) 55(2) -9(1) -21(1) -2(1) F(3) 87(1) 99(2) 71(1) -24(1) -38(1) -24(1)

137 APPENDIX

A.3. CALCULATION OF THE EFFECTIVE CONE ANGLE

The Tolman cone angle used to quantify the steric effect of the ligand is determined using Eq.

A.3-1, where θt is the apex angle of a cylindrical cone, centred 2.28 Å from the centre of P atom, to the van der Waals radii of the outermost atoms of the model [1]. Figure A.3-1 below illustrates the argument used. But Roodt [2] and co-workers introduced an effective cone angle, in which the apex angles are measured from the metal centre to the outermost atom on the substituents using observed M-P bond length obtained crystallographically.

r P d

2.28 Å

θc θm θt M

Figure A.3-1 The method of determining cone angles for symmetrical ligands. Here r is the van de Waal radii of hydrogen atoms (1.2 Å) [3].

For a symmetrical ligand PX3, the effective cone angle is defined using Eq. (A.3-1) below.

θ = 2 θ (A.3-1) E 3 ∑ t 1−3

1 Tolman A. C., Chem. Rev., 1977, 77, 3, 313. 2 Roodt A., Otto S., Smith J., Inorg. Chim. Acta., 2000, 303, 295. 3 Smith J M., Coville N. J., Organometallics, 2001, 20, 1210.

138 APPENDIX

The value of θt is the sum of θc and θm, here θc is the angle illustrated in Figure A.3-1 above and θm is the measured angle between H(x)-M-P.

θt = θc + θm (A.3-2)

The value of θc is determined by trigonometry as shown in Eq. (A.3-4) below.

ο 1.2A Sin θc = d (A.3-3)

ο 1.2A θc = Sin-1 d (A.3-4)

5 For [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}] the θc values are calculated below from respective d values, where d is the distance from the metal to the hydrogen atom in question from structural data.

For H(12) with d equal to 3.679 Å (see Table A.3-1) the value of θc is determined as indicated below.

-1  1.2  θc = Sin   = 19.04˚  3.679 

θt is then determined using equation (A.3-2) above. In this case the value of θm is equal to 49.62˚.

θt = 19.04 + 49.62 = 68.66˚

For each substituent the proton with the highest θt value (outermost proton) is used to determine the overall cone angle or the effective cone angle, θE, using Eq. (A.3-1) mentioned above.

(228.91 x 2) θE = 2 (68.66 + 74.09 + 86.16) = = 152.61˚ 3 3

139 APPENDIX

5 Table A.3-1 below compiles the values of [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}] that were used to determine the effective [2] cone angle.

Table A.3-1 Table of values for determination of effective [2] cone angle of [(η-C5H5)Fe(CO)(COMe){P(p- MePh)3}]. 1.2 is the van de Waal radii for hydrogen atoms.

θm Length θt (˚) Highest θt (˚) (Å) 1.2 value H H (˚)1.2 H12 49.62 H12 3.679 68.66 H13 53.28 H13 5.750 65.33 H15 39.36 H15 6.353 50.25 H16 32.30 H16 4.602 47.41 H17A 54.10 H17A 7.600 63.18 H17B 42.89 H17B 7.977 51.54 H17C 48.9 H17C 7.755 57.80 68.66

H22 54.23 H22 3.533 74.06 H23 58.23 H23 5.583 70.64 H25 40.00 H25 6.326 50.93 H26 30.35 H26 4.616 45.42 H27A 49.28 H27A 7.790 58.14 H27B 48.34 H27B 7.809 57.18 H27C 58.49 H27C 7.443 67.77 74.09

H32 13.33 H32 4.979 27.28 H33 28.17 H33 6.652 38.56 H35 63.24 H35 5.410 76.06 H36 63.51 H36 3.116 86.16 H37A 44.03 H37A 7.962 52.70 H37B 55.09 H37B 7.570 64.21 H37C 48.36 H37C 7.781 57.23 86.16

140 APPENDIX

Similar arguments were used to determine the effective [2] cone angle of the [(η5-

C5H5)Fe(CO)(COMe){P(p-FPh)3}] complex and the Table of values used is presented below in Table A.3-2.

Table A.3-2 Table of values used to determine the effective [2] cone angle of [(η- C5H5)Fe(CO)(COMe){P(p-FPh)3}]. 1.2 is the van de Waal radii for hydrogen atoms. H θm H Length θt Highest θt (˚) (Å) (˚) 1.2 value (˚) 1.2 H12 63.86 H12 3.06 86.95 H13 64.18 H13 5.363 77.11 H15 29.49 H15 6.646 39.89 H16 14.62 H16 4.995 28.52 F1 49.16 F1 7.145 58.83 86.95

H22 47.77 H22 3.799 66.18 H23 53.38 H23 5.778 65.37 H25 44.73 H25 6.171 55.94 H26 36.39 H26 4.408 52.19 F2 50.60 F2 7.084 60.35 66.18

H32 55.33 H32 3.477 75.52 H33 58.71 H33 5.580 71.13 H35 38.61 H35 6.394 49.43 H36 27.81 H36 4.695 42.62 F3 50.12 F3 7.120 59.82 75.52

(228.65 x 2) θE = 2 (86.95 + 66.18 + 75.52) = = 152.43˚ 3 3

141 APPENDIX

A.4. EQUATIONS OF LEAST-SQUARES PLANES IN COMPLEX A AND B

Table A.4-1 (Table 3-9) Equation of least-squares planes for [(η-C5H5)Fe(CO)(COMe){P(p-MePh)3}], A. Plane 1: defined by Cp ring -6.7373x - 0.0228y + 10.2606z = 3.4835 Plane 2: defined by P1, C1, C2 -6.7224x - 1.1071y + 10.2229z = 0.1881 Plane 3: defined by C1, O1, C2, C3 -5.0441x + 6.1567y + 6.7878z = 3.4149 Plane 4: defined by Phenyl ring 1 4.0006x - 8.9341y + 5.2493z = 0.6069 Plane 5: defined by phenyl ring 2 -5.2754x + 2.2248y + 12.0881z = 1.8736 Plane 6: defined by phenyl ring 3 5.2429x + 6.3742y - 3.9523z = 2.1160

Table A.4-2 (Table 3-11) Equation of least-squares planes for [(η-C5H5)Fe(CO)(COMe){P(p-FPh)3}], B. Plane 1: defined by Cp ring -6.7073x - 3.4949y + 3.7906z = 0.2837 Plane 2: defined by P1, C1, C2 6.5312x + 4.5113y - 2.3321z = 3.1968 Plane 3: defined by C1, O1, C2, C3 -5.4986x + 0.3411y + 9.9576z = 1.3571 Plane 4: defined by phenyl ring 1 4.9849x + 6.3869y + 10.0011z = 5.7079 Plane 5: defined by phenyl ring 2 -4.0447x + 6.2783y + 7.5818z = 0.6065 Plane 6: defined by phenyl ring 3 4.9999x + 2.6649y - 9.3122z = 0.5888

142 APPENDIX

A.5. DERIVATION OF THE RATE LAWS

Different possibilities used to interpret the data are presented below with full derivation.

A.5.1. Formation of the Outer-sphere Intermediate Species

K1, k1 [(η5−CnHn)Fe(CO)2(Me)] + PX3 [(η5−CnHn)Fe(CO)2(Me), {PX3}]# k-1 k 5 # 2 5 [(η −CnHn)Fe(CO)2(Me), {PX3}] [(η −CnHn)Fe(COMe)(CO){PX3}]

5 Scheme A.5-1 Proposed pathway that migratory insertion follows [4]. (η -CnHn) is either a Cp or indenyl ring.

5 # Fe refers to the alkyl complex, [(η -CnHn)Fe(CO)2(Me)], Fe,L is the intermediate species, 5 # 5 [(η -CnHn)Fe(CO)2(Me),{PX3}] , [Fe(COMe)] is the final product [(η -

CnHn)Fe(COMe)(CO){PX3}] and PX3 is L. K1 is the equilibrium constant for the formation of the activated species. k2 is the rate constant for the methyl migration step.

(i) In Scheme A.5-1 Above pre-equilibrium step precedes the second step, which is the rate- determining step. The rate of the reaction is determined as follows.

d[Fe(COMe)] Rate = = k [Fe,L# ] (A.5-1) dt 2

Here [Fe(COMe)] is the concentration of the product, [Fe,L#] is the concentration of the outer-sphere intermediate complex and k2 is the rate constant for the methyl migration step.

# [Fe]Tot = [Fe]Rem + [Fe,L ] (A.5-2)

4 Monti D., Bassetti M., J. Am. Chem. Soc., 1993, 115, 4658.

143 APPENDIX

[Fe,L# ] K1 = (A.5-3) [Fe]Rem [L]

K1 is the equilibrium constant for the formation of the activated species and [L] is the concentration of the phosphine ligand in the reaction mixture.

[Fe,L# ] [Fe]Rem = (A.5-4) K1[L]

# [Fe,L ] # [Fe]Tot = + [Fe,L ] (A.5-5) K1[L]

K [L][Fe] [Fe,L# ] = 1 Tot (A.5-6) 1+ K1[L]

Rearrangement of Eq. (A.5-2 and A.5-3) and substitution into Eq. (A.5-1) gives the rate constant as Eq. (A.5-7) illustrates. Integration of the rate equation below (Eq. A.5-7), gives the equation of the observed rate constant as is illustrated in Eq. (A.5-8).

k K [L][Fe] k = 2 1 Tot (A.5-7) 1 + K1[L]

k2 K1[L] kobs = (A.5-8) 1 + K1[L]

(ii) If the first pre-equilibrium step is the rate-determining step then the rate of the reaction is given by Eq. A.5-9.

d[Fe(COMe)] Rate = = k [Fe,L# ] (A.5-9) dt 2

144 APPENDIX

By applying steady state approximation and integration the rate of the reaction is given by Eq. (A.5-13) below.

d[Fe,L# ] Rate = = 0 = k [Fe][L]− k [Fe,L# ] − k [Fe,L# ] (A.5-10) dt 1 −1 2

Here k-1 is the rate constant for the reverse reaction of the outer-sphere complex formation. Rearrangement of components gives Eq. (A.5-12).

# # k1[Fe][L] = k−1[Fe,L ] + k2 [Fe,L ] (A.5-11)

k [Fe][L] [Fe,L# ] = 1 (A5-12) k−1 + k2

Substituting Eq. (A.5-12) into Eq. (A.5-9) gives the expression for the rate of the reaction, (Eq. (A.5-13)).

k k [Fe][L] k = 2 1 (A.5-13) k−1 + k2

Upon integration the observed rate constant equation is given by (Eq. (A.5-14)) below.

k2 k1[L] kobs = (A.5-14) k−1 + k2

kobs is the pseudo first order rate constant of the reaction.

1 kobs = K [L] (A.5-15)

Another proposed mechanism involves methyl migration followed by nucleophilic attack on the intermediate according to Scheme A.5-2 below.

145 APPENDIX

A.5.2. Formation of the Acyl Intermediate Species

K1, k1 [(η5−CnHn)Fe(CO)2(Me)] + S [(η5−CnHn)Fe(COMe)(CO)S]# k-1 k 5 # 2 [(η −CnHn)Fe(COMe)(CO)S] + PX3 [(η5−CnHn)Fe(COMe)(CO){PX3}] -S

5 Scheme A.5-2 The second proposed mechanism for migratory insertion [5]. (η -CnHn) is either a Cp or an indenyl ring.

5 # 5 # 5 Fe refers to [(η -CnHn)Fe(CO)2(Me)], Fe is [(η -CnHn)Fe(COMe)(CO)S] [(η -

CnHn)Fe(COMe)(CO){PX3}] is [Fe(COMe)] and PX3 is L.

(i) If the rate-determining step is the pre-equilibrium step, the rate of the reaction (Scheme A.5-2) is given by the equations that follow.

d[Fe(COMe)] Rate = = k [Fe# ][L] (A.5-16) dt 2

Here k2 is the rate constant for phosphine coordination step, [Fe(COMe)] is the concentration of the final product, [Fe#] is the concentration of the intermediate species and [L] is the concentration of the phosphine ligand.

Assuming that the intermediate is at steady state, integration of the rate law gives the observed rate constant as Eq. (A.5-20).

d[Fe# ] Rate = = 0 = k [Fe] − k [Fe# ] − k [Fe# ][L] (A.5-17) dt 1 −1 2

Here k-1 is the rate constant of the decarbonylation reaction and [Fe] refers to concentration of the alkyl complex.

5 (a) Mawby R. J., Basolo F., Pearson R. G., J. Am. Chem. Soc., 1964, 86, 3994, (b) Butler I. S., Basolo F., Pearson R. G., Inorg. Chem., 1967, 6, 2074.

146 APPENDIX

k [Fe] [Fe# ] = 1 (A.5-18) k−1 + k2 [L]

Substitution of Eq. (A.5-18) into Eq. (A.5-16) gives the rate of the reaction as illustrated below (Eq. (A.5-19)).

k k [Fe][L] k = 2 1 (A.5-19) k−1 + k2 [L]

Since [L] >>> [Fe] then integration of the rate equation (Eq. A.5-19) gives the observed rate constant as illustrated by (Eq. A.5-20) below.

k2 k1[Fe][L] kobs = (A.5-20) k−1 + k2 [L]

(ii) In the other case the rate-determining step is the second step, which involves the attack of the acyl intermediate by a phosphine ligand. In that case the rate of the reaction is determined as follows.

d[Fe(COMe)] Rate = = k [Fe# ][L] (A.5-21) dt 2

# [Fe]Tot = [Fe]Rem + [Fe ] (A.5-22)

[Fe# ] K1 = (A.5-23) [Fe]Rem

Here K1 is the equilibrium constant for the formation of the acetyl intermediate species.

Rearrangement of Eq. (A.5-23) followed by substitution into Eq. (A.5-22) gives Eq. (A.5-25).

147 APPENDIX

[Fe# ] [Fe]Rem = (A.5-24) K1

# [Fe ] # [Fe]Tot = + [Fe ] (A.5-25) K1

Rearrangement of Eq. (A.5-25) yields Eq. (A.5-26), which gives the observed rate constant (Eq. (A.5-27)) when substituted into Eq. (A.5-21).

K [Fe] [Fe#] = 1 Tot (A.5-26) 1 + K1

K1k2 [L] kobs = (A.5-27) 1+ K1

1 kobs = K [L] (A.5-28)

A.6. SUMMARY OF METHODS USED TO DRY SOLVENTS

A.6.1. Dichloromethane

Dichlomethane was refluxed for three hours on calcium chloride under a nitrogen blanket. This was then collected on dry molecular-sieves.

A.6.2. Acetonitrile

Acetonitrile was distilled from small amounts of phosphurus pentoxide (P2O5) for several hours under a nitrogen blanket leaving behind one fifth of the nitrile inside the distilling flask. It was then stored on dry molecular sieves.

148 APPENDIX

A.7. SUPPLEMENTARY DATA FOR P(p-MePh)3

5 Table A.7-1 (Figure 4-1) FT- IR data for the reaction between [(η -C5H5)Fe(CO)2Me] (31 mM) and [P(p- 5 MePh)3] (140 mM) in MeCN at T = 58.0 ˚C, yielding [(η -C5H5)Fe(CO)(COMe){P(p-MePh)3}]. A and B refer to the bands of the starting alkyl complex [2] and C is the band of the acyl product. Time Absorbance Absorbance Absorbance (hrs.) A a) B b) C c) 0.1 0.113 0.115 0.007 15 0.085 0.087 0.024 24 0.066 0.072 0.037 47 0.037 0.044 0.055 72 0.024 0.032 0.061 88 0.023 0.031 0.064 a) band at 2006 b) band at 1946 c) band at 1915

149 APPENDIX

Absorbance

10-3 Time (sec) 5 Figure A.7-1 (Table 4-1) IR kinetic plots for the reaction between [(η -C5H5)Fe(CO)2Me] (31 mM) and 5 [P(p-MePh)3] (140 mM) in MeCN at T = 58.0 ˚C, yielding [(η -C5H5)Fe(CO)(COMe){P(p- MePh)3}]. A and B refer to the bands of the starting alkyl complex [2] and C is the band of the acyl product. This are plots of data in Table A.7-1.

150 APPENDIX

A.8. SUPPLEMENTARY DATA FOR P(p-FPh)3

5 Table A.8-1 (Figure 4-8) FT-IR data for the reaction between [(η -C5H5)Fe(CO)2Me] and P(p-FPh)3 5 5 yielding [(η -C5H5)Fe(CO)(COMe){P(p-FPh)3}], [P(p-FPh)3] = 120 mM and [(η - C5H5)Fe(CO)2Me] = 31 mM in MeCN. Sample withdrawal was done at time intervals displayed 5 on the Figure in hours. Bands A and B are the bands for [(η -C5H5)Fe(CO)2Me], C and D are the bands for the product. T= 58.0 ˚C. Time Time Absorbance Absorbance Absorbance (sec.) (hrs.) A a) B b) C c) 18720 5.20 1.24 1.42 0.10 70200 19.50 0.92 1.08 0.20 189960 52.75 0.58 0.72 0.38 337680 93.80 0.26 0.36 0.46 547320 152.03 0.26 0.34 0.54 636900 176.91 0.22 0.30 0.62 720120 200.03 0.21 0.28 0.75 a) Band at 2006 b) Band at 1946 c) Band at 1918

19 5 Table A.8-2 (Figure 4-10) F NMR data for the reaction between [P(p-FPh)3], A (120 mM) and [(η - 5 C5H5)Fe(CO)2Me] (31 mM) yielding [(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}], B, in MeCN at T = 58.0 ˚C. C is the corresponding phosphine oxide. kobs obtained is given in Table 4-6.

Time P(p-FPh)3 [(Cp)Fe(CO)COMe{L}] OP(p-FPh)3 (sec.) A B C 170 99.12 0 0.88 29750 93.4 5.66 9.43 77210 77.67 9.21 13.15 113210 67.31 13.46 19.2 168230 57.89 15.79 26.31 274630 46.42 17.86 35.71 361610 36.36 18.18 45.45 427370 7.69 15.38 76.92 1133600 0 16.67 83.3 1216850 0 9 91 1382400 - - -

151 APPENDIX

19 5 Table A.8-3 (Table 4-6) F NMR data for the reaction between [P(p-FPh)3], A (120 mM) and [(η - 5 C5H5)Fe(CO)2Me] (31 mM) yielding [(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}], B, in MeCN at T = 58.0 ˚C. C is the corresponding phosphine oxide. kobs obtained is given in Table 4-6.

Time P(p-FPh)3 [(Cp)Fe(CO)(COMe){L}] OP(p-FPh)3 (sec.) A B C 120 90.75 4.13 5.11 27120 87.92 5.67 6.42 76060 80.83 10.88 8.29 102300 75.44 13.04 11.52 156420 64.96 15.71 19.33 182640 60 17.28 22.72 240000 51.78 18.49 29.73 329640 44.17 20.09 35.74 415440 37.82 20.97 41.21 502260 35.73 19.11 45.16 588180 32.11 19.13 48.76

19 5 Table A.8-4 (Table 4-6) F NMR data for the reaction between [P(p-FPh)3], A (210 mM) and [(η - 5 C5H5)Fe(CO)2Me] (31 mM) yielding [(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}], B, in MeCN at T = 58.0 ˚C. C is the corresponding phosphine oxide. kobs obtained is given in Table 4-6.

Time P(p-FPh)3 [(Cp)Fe(CO)(COMe){L}] OP(p-FPh)3 (sec.) A B C 240 96.32 0 3.68 29580 91.76 4.58 3.66 79500 84.49 9.01 6.5 105660 82.51 9.69 7.8 159840 78.38 11.65 11.97 186000 76.13 10.79 13.08 243240 69.44 13.68 16.98 333000 63.69 12.66 23.66 418140 61.49 12.19 26.32 505020 54.64 13.86 31.5 590880 53.73 11.69 34.57

152 APPENDIX

19 5 Table A.8-5 (Table 4-6) F NMR data for the reaction between [P(p-FPh)3], A (270 mM) and [(η - 5 C5H5)Fe(CO)2Me] (31 mM) yielding [(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}], B, in MeCN at T = 58.0 ˚C. C is the corresponding phosphine oxide. kobs obtained is given in Table 4-6.

Time P(p-FPh)3 [(Cp)Fe(CO)(COMe){L}] OP(p-FPh)3 (sec.) A B C 240 97.17 0 2.83 53460 92.4 3.38 4.22 64260 91.39 3.38 4.78 75060 90.27 4.32 5.4 85920 89.09 4.85 6.06 143520 84.3 6.957 8.695 154320 83.96 6.6 9.43 165120 83 7.07 10.1 171900

19 5 Table A.8-6 (Table 4-6) F NMR data for the reaction between [P(p-FPh)3], A (340 mM) and [(η - 5 C5H5)Fe(CO)2Me] (31 mM) yielding [(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}], B, in MeCN at T = 58.0 ˚C. C is the corresponding phosphine oxide. kobs obtained is given in Table 4-6.

Time P(p-FPh)3 [(Cp)Fe(CO)(COMe){L}] P(p-FPh)3 (sec.) A B C 269 98.2 0 1.75 19121 96.96 0.8696 2.174 72461 94.52 2.594 2.882 84941 93.62 3.175 3.175 108101 - - - 158021 90.58 4.933 4.484 178721 89.94 4.76 5.29 188861 - - - 360761 81.08 5.4 11.82 424220 80 4.62 13.45

3 -6 -1 10 kobs s 2

0 0.00 0.070.14 0.21 0.28 0.35 [P(p-FPh) ] (M) 3 19 Figure A.8-1 The F NMR kinetic plot showing the dependence of kobs on the [P(p-FPh)3] for the formation of the P(p-FPh)3 oxide, C from solutions of a range of concentration of P(p-FPh)3 and 5 [(η -C5H5)Fe(CO)2Me] (31 mM) at T = 58.0 ˚C in MeCN. “•” a point from a different kinetic run.

153 APPENDIX

31 5 Table A.8-7 (Figure 4-13 and 4-14) The P NMR data for the reaction of [(η -C5H5)Fe(CO)2Me] (30 mM) 5 and [P(p-FPh)3], A, (120 mM) in MeCN yielding [(η -C5H5)Fe(COMe)(CO){P(p-FPh)3}], B. T = 58.0 ˚C.

Time P(p-FPh)3 [(Cp)Fe(CO)(COMe){L}] OP(p-FPh)3 (sec.) A B C 360 97.2 0 2.8 18720 82.8 5.38 11.83 70200 - - - 104760 61.52 15.57 22.91 157620 54.29 14.29 31.43 189960 55.81 13.94 30.24 244980 55.36 13.64 31

154 APPENDIX

Absorbance

10-3 Time (sec)

5 Figure A.8-2 (Table 4-8) UV-visible kinetic plots of for the reaction between [(η -C5H5)Fe(CO)2Me] (52 mM) and a range of concentrations of P(p-FPh)3 (A = 5, B = 41, C = 79 and D = 150 mM) in MeCN at T = 50.0 ˚C. λ = 360 nm.

155 APPENDIX

A.9. SPECTROSCOPY DATA

Table A.9-1 Summary of carbonyl stretching frequency obtained from the IR analysis of the product.

P-donor ligand νCO νCO (cm-1) a) (cm-1) b) PPhCy2 1912 1916 P(p-MeOPh)3 1912 1919 P(p-MePh)3 1915 1920 PPh3 1916 1922 P(p-FPh)3 1918 1924 c) CpFe(CO)2Me 2006, 1946 a) Experimental-stretching frequencies in Chloroform at room temperature (This study). b) Literature [6] stretching frequencies in cyclohexane at ambient temperature. c) Cp is -C5H5

Table A.9-2 Table of summary of 1H and 31P chemical shift of the products and the starting material. 1H chemical shifts δ 31P δ (ppm) (ppm) P-donor ligand Cp Me Ph PPhCy2 4.39 2.67 7.39-7.90 72.27 P(p-MeOPh)3 4.41 2.32 6.86-7.43 71.34 P(p-MePh)3 4.38 2.33 7.13-7.16 73.68 PPh3 4.40 2.30 7.35-7.80 76.24 P(p-FPh)3 4.39 2.32 7.05-7.68 75.40 CpFe(CO)2Me 4.73 0.14 - -

6 Rahman M., Liu H., Eriks K., Prock A., Giering W P., Organometallics, 1989, 8, 1.

156